Harry Benson does not believe in treating photography as art, but even he could not resist hanging his most famous photograph in his sitting room. It shows the Beatles pillow-fighting on the night they discovered they had their first number one single in the US: a defining moment for both the band and Benson, then a 34-year old Fleet Street photographer who undertook the assignment only at the insistence of his editor. The fantastic photo, featured on the previous page, is infused with energy and so effortlessly spontaneous that you need to be reminded that it was all contrived by Benson at 2am in the morning. John Lennon, apparently, thought it was a silly idea but played along anyway.

Lennon complained again a few weeks later when the band flew to New York to record the Ed Sullivan Show and Benson set them up with Cassius Clay shortly before he beat Sonny Liston to become heavyweight champion of the world . Yet the photographer's unerring instinct to create images that define their era overcame his subjects' unease, and the shots of Clay sparring with the Beatles now seem as iconic as that pillow fight.

This month the veteran photographer publishes Once There Was a Way, a book of his Beatles photographs capturing the tumultuous years from 1964 to 1966. ''I didn't imagine 40 years ago that I'd be putting a picture of the Beatles on my wall and writing a book about them,'' Benson tells me as his wife, Gigi, ferries pots of tea and plates of shortbread from one end of the coffee table to the other. ''I was just hoping to stay on the payroll at the end of the week.'' He probably didn't imagine that said wall would belong to an elegant Upper East Side apartment in Manhattan either. Then again, a lot of things that have happened in Benson's life would once have seemed unimaginable: standing near Bobby Kennedy the night he was assassinated; being with Nixon the day he resigned; being in Berlin the day the wall went up and there again to watch it come down; being in New York on the morning of September

11. In retrospect, so much of it seems like serendipity, but that is only part of the equation. What Benson calls ''going for the centre of a story'' - and others might call hunger - is equally important. He would not have caught the pillow fight had he not stayed up, waiting for the right moment, then spent the rest of the morning in the toilet of his room at a Paris hotel developing his prints. ''I messed up more toilets with chemicals than I can count,'' he chuckles. ''People probably got skin diseases from the rooms I was in.''

Benson hardly seemed pre-ordained for a career as 20th-century chronicler. As one of four children growing up in Milngavie, just outside Glasgow, in the thirties and forties, he developed an early sense of inferiority, forced to leave school at 13 by a system that sorted children into failures and successes before they had even reached puberty. ''My school days were miserable, miserable,'' says Benson, who claims his difficulty in mastering classical languages was his school's self-serving justification for booting him out. Instead he found work as a messenger boy, pushing a barrow around town and building up a head of resentment and anger. He remembers going home on spring evenings and trying to avoid the eyes of his old classmates. ''You feel such a misfit, a loser, and no one really wants much to do with you, except other stupid people like you,'' he says. ''I always thought it was a cruel

thing.''

It is a paradox, perhaps, that Benson's drive and ambition would spring from the experiences that left him so deflated as a child. He tells a revealing anecdote about a class assignment he was given: to write an essay on Little Women, which he had not bothered to read. ''So I wrote about a little woman who came to help my mum clean our house, and after she finished she would jump the fence and help the woman next door.'' Instead of failing him for it, his perceptive English teacher, Miss McKenzie, invited him to read it to the class. ''Everyone was roaring with laughter, and she said, 'Stop, let me tell you all something about Benson: he's going to finish up better than any of you.' I always remember that woman saying that. It was a very nice thing to say.''

Miss McKenzie knew what she was talking about. The initiative and imagination that Benson showed in his essay would fuel his subsequent career, propelling the school reject from one job to another and ultimately to America: a place where opportunities did not depend on your accent or which school you went to. ''It was always very much about the status quo in Britain,'' he says.

With hindsight it makes sense that the boy who could not master languages learned to master pictures instead, but his success did not come easily. ''I never trusted an easy life,'' he says. ''I've always tried to put my boat straight into the wind, but it took me a long time to get started. I mean, I did weddings in Glasgow - terrible weddings, the kids from the tough neighbourhoods - and I'd go back to a hut to develop the film before taking the prints back to the reception. A few times I came back and they'd all been drinking and they'd fight; I mean, really fight.''

From weddings he graduated to a summer stint at Butlins in Ayr, before landing a job as a staff photographer on the Hamilton Advertiser. Soon he was taking the sleeper to London and touting for jobs at the nationals until eventually the Daily Sketch gave him an assignment. He was at the Advertiser one afternoon when his mother called to tell him to phone the Daily Sketch picture desk. ''They gave me a few jobs and eventually I covered Scotland, which was the best job I ever had,'' he recalls. It was during this time that Benson showed some of the initiative that would make him one of the best photographers on Fleet Street. An unsavoury friend, a boxing promoter, offered to put him in touch with the Scottish mass murderer Peter Manuel, then in jail awaiting trial. Deluged with requests for press interviews, Manuel had given a list of reporters' names to a warden; Benson cannily struck

up a rapport with this warden and convinced him to cross out all the other names. ''It turned out that the only people Manuel saw were his priest, his mother and me,'' says Benson, laughing. ''Manuel said, 'Harry, I want to thank you very much for being the only one who comes. I don't understand these people; they send me these begging letters and I never hear from them again.'''

Not long after, Benson came second in a competition organised by the Encyclopaedia Britannica to find the best photographer of the year, and the Daily Sketch brought him down to London. He had not been there long when he gatecrashed a party thrown by Max Aitken, son of Lord Beaverbrook, who promptly offered him a job at The Daily Express and then forgot about him for three months. Benson was covering an event at the Caprice restaurant in London when he spotted Aitken again, and cornered him. ''I said, 'I've been hoping to get a call from you' and he said, 'You'll get one tomorrow'.'' For years Benson wondered why he had been held back, but that mystery was solved recently by a retired picture editor who told him that the paper had asked around Fleet Street to see if Benson was reliable. ''Apparently word came back saying, 'He's a good one, but he's trouble,''' says Benson, grinning like a

monkey. What kind of trouble? ''Well, let's just say that on my first day they sent me down to Chelsea to do a story on the Chelsea hippies, and I'm there with a reporter called Jeremy Banks. A big American car pulls up and a guy leans out and says, 'I've been looking for you, Banks,' and starts to slap him. So I pull him out and belt him.'' He belted him so hard, in fact, that he broke his hand and had to go to hospital to have it bandaged. He made the doctor promise not to tell his employers - only to find out that his employers already knew, and were dispatching a lawyer to help him.

Benson was due to accompany a reporter to Kenya to mark the nation's first year of independence when he was given the assignment to cover the Beatles in Paris. He readily concedes that he had only a dim knowledge of the band, and felt loath to give up his African adventure to trail after a pop group. He soon changed his mind, though. ''Imagine me standing onstage next to Paul McCartney, and he says to me, 'We want you to cheer for us, Harry.' And then I went offstage and watched them play All My Loving, and it was incredible. It sold me right away. I think I got with them at the right time, certainly of my life but also of their life, right at the beginning, because they didn't really know what was going to happen.''

It is hard to imagine there was ever a time when people didn't know who the Beatles were, but when Benson hooked up with them early in 1964, Beatlemania was still a largely British phenomenon. He was able to photograph them on the Champs Elysees without risk of being mobbed - something that would be impossible by April, when the band stormed the US charts. ''You knew they were something special, really extraordinary, as they proved to be,'' he says. ''I think they had the same impact on our culture as Hitler; they crossed every line of our culture.''

Perhaps because it was still early days, those first pictures have a carefree quality absent from later photographs: clowning around with French customs officials, drinking Pepsis in a bar, mugging for the camera on the flight to New York. When they arrived at Idlewild, as JFK airport was then known, Benson made sure they remembered to turn around and wave as they walked down the steps to the waiting press. It was another iconic moment, selected by Life magazine last year as one of 100 photographs that changed the world. In fact it did no such thing and was almost lost to the world: unimpressed by the crowd, who had been held back by police, Benson tossed the film into his closet, where it lay undeveloped for the next 20 years.

When Benson joined the group for their second (and last) American tour in 1966, things were already on the slide. Exhausted by a punishing tour schedule, the band seemed more sombre than Benson had remembered from previous encounters. John's off-the-cuff remark to a British journalist that the Beatles were more popular than Jesus had provoked a furious response, and Benson arrived to find a tearful John nursing his pride after being forced to apologise. The photograph of a downcast Lennon, cigarette in hand and what appears to be a saucer of aspirin nearby, neatly captures the slow dissolution of the band. They were increasingly fractious (''basically they were arguing with Paul,'' says Benson), and a year later Brian Epstein would be dead, leaving the band without a manager. ''I liked Epstein,'' Benson says softly. ''He was a very good guy. I wish I'd said goodbye to him.''

Although he developed a casual friendship with the band, especially with Harrison, who shared a room with him in Paris, Benson resisted getting too close, a rule that has guided his career ever since. ''I never want to be with anyone that I'm shooting for very long because it creates a different relationship,'' he says. ''If a celebrity invites me to dinner I never go - I just don't want them to figure me out, or to say afterwards, 'Oh Harry, that picture of me having a bath, please don't use it'. Bang goes your best picture in a bath because of a friendship. This is a problem with younger journalists and photographers. They think they've become friends with people, and they haven't. You could be lying dead in the street and they'd just step over you.''

As a rule, Benson likes his subjects mean and taciturn. ''When someone tells me that so-and-so is a wonderful human being, I know I've got a problem,'' he says. ''If I've got a real piece of shit who everybody really hates, now I've automatically put my artillery in the right position to do the job. I'm prepared for them.'' He was more than ready for Richard Nixon, who turned out to be his favourite president of the eight he has photographed. He accompanied him to Jerusalem in 1974, and was on hand a few months later to capture him, clench-jawed, as he resigned the presidency. ''He let me into San Clemente a few days after leaving office, and when the shoot was over I said, 'I want to thank you, Mr President, because I know this isn't the best time of your life', and he said, 'Harry, you've got to allow professional people to do their job.''' It was exactly the kind of pragmatic sentiment

that Benson applied to his own life.

Benson's hard-boiled approach proved particularly useful in 1968 as he followed Bobby Kennedy on the campaign trail. By that stage he was living permanently in the US, having clocked up some impressive coups, including searing images of the Watts riots in Los Angeles and civil rights battles in Mississippi. Dispatched to the Ambassador Hotel in LA on June 4 to cover Kennedy's victory speech after winning the California primary, he found himself photographing his shooting instead. While other photographers held back, Benson clambered on top of a stove and started snapping aerial shots of the senator's body, saying under his breath: ''Pull yourself together, don't blow it, don't fail. Stay at the centre. This is for history.'' It was a night that still haunts him, but his photographer's instincts prevailed and he got his scoop again.

Of all the subjects Benson has photographed, only a few have caused him genuine distress. ''I was always uncomfortable in Africa,'' he says of the trips he has made documenting civil war and famine. ''It made me awfully uncomfortable to photograph the little fat babies dying of starvation; I always thought you took away their dignity. And other photographers would wait until there was good lighting so they got a better picture - that always disturbed me.'' He also makes it a point not to photograph animals in degrading situations.

Perhaps because his trade is to tell stories with pictures, not words, Benson seems oblivious to his capacity to entertain. He will be in the middle of telling you about the Watts riots, or how he secretly photographed Greta Garbo, or which Beatle received the most fan mail (Ringo, since you ask) when he will abruptly pause to throw a quizzical look in your direction. ''Is this all right?'' he'll fret. ''I'm not boring you, am I?'' He seems to require a lot of reassurance on this point, before plunging back into the thickets of his memory. ''So, like I said to Henry Kissinger the other week '' And we are off into another anecdote from a life that has played witness to a half century of history.

You would think that, at 74, with five decades of work to his name, he might be ready to relax, but not a bit of it. He has just been shooting the film-maker James Ivory; and when I send an e-mail with a follow-up question a few days after we meet, Gigi responds to say that he is in Florida on assignment for Vanity Fair. He is among the most prolific photographers in the business, and shows no desire to stop. ''I think in life you've got to do what make you happy,'' he tells me, and although it is an obvious thing to say, it does seem to distil the essence of Benson's success. He remembers listening to Churchill's wartime speeches and wanting desperately to be at the heart of life, not relegated to the sidelines.

Although he is now a naturalised American citizen, Benson has lost neither his wry humour nor his Glasgow brogue; and although his best work is born from a steely resolve, there is a warmth and camaraderie about him that must come in useful when he needs to put a celebrity at ease. He tells me how he persuaded Ronald and Nancy Reagan to dance for him at the White House, for one of Vanity Fair's more celebrated covers, and it becomes clear that he is as much a director as a photographer. Despite the apparent spontaneity of his pictures, there is a lot of stage management in his work - often without the players' knowledge.

As I am about to leave, Benson digs out a fantastically sinewy walking stick and explains that it once belonged to Harry Lauder, the Scottish music-hall legend. It was given to him by his father, who perhaps understood better than most the theatrical qualities at the heart of his son's profession. For all that Benson abhors pretentious talk, there is both art and entertainment in his best shots, which in turn ensures their enduring power. Like Lauder's walking stick, time has turned them into cultural artefacts.

''I'm still driven by the uncertainty of it all,'' says Benson as he walks me to the elevator. ''Sometimes people say, 'There's not much in it,' and I always think, 'How do you know there's not much in it?' Because, until you walk into a room, how do you know?'' n

Once There Was a Way is published by Thames & Hudson, priced (pounds) 19.95. For more of Benson's work, visit www.harrybenson.com