"I am a camera." The phrase belongs to Christopher Isherwood - but these could almost have been Jacques Henri Lartigue's first, and last, words. Dozens of his photographs fill the walls at the Hayward Gallery in London; thousands more are displayed in row upon row of vitrines. What image to start with? Should it be balloonists and early powered flights, photographed before the first world war? Or that time he pointed his camera at the TV (as so many people did) and shot a roll of images of men walking on the moon? How about a honeymoon portrait of his first wife on the toilet, in 1920? What about all those women that he saw on their afternoon promenade in the Bois de Boulogne, after whom he hankered as an adolescent? Or this sinister moment on the street during the last days of the German occupation, which Lartigue surreptitiously shot from his window in 1944?  Here is an image, almost at random. The heavy furniture looms over the low horizon of the carpet. What really matters in the photograph occurs on the thin strip of floor at the bottom of the picture. "In my bedroom, my collection of racing cars," the handwritten caption reads, "40 rue Cortambert, Paris 1905." The little painted tin and lead cars are lined up and ready to go. I imagine their owner, 11 years old and already a seasoned photographer, arranging his prize possessions and pointing his camera - an even more valued object - across the floor at carpet level. A mirror above gives an accidental, dizzying view of the apartment's far ceiling.  What the photograph does not show is the boy's look of happiness and concentration - though it is difficult not to imagine it - as he surveys his toys. Already he has an idea of speed in his head: the speed of the internal combustion engine, the speed of photography, the rush of life in the new century. Blink and it's gone. That singular moment, one of so many moments, from a life in photography that spanned more than 80 years; all those moments brought to life through the photograph's total recall.  Years later, driving the family's enormous and powerful Hispano-Suiza (the only one in France), did Lartigue remember that childhood image? Of course he did, because he would have had even his earliest photos to hand, in one of his albums. He kept everything, photographed everything, the camera always with him. Perhaps it was not so much that Lartigue spent his life taking photographs, as that he photographed his life, in order to keep hold of it, to prove that it was real. Isn't this why we all get the camera out for family gatherings, for birthdays, holidays and weddings, for a child's first steps?  For Lartigue, this need was more acute. One day, as a child, the delicate youngest son of the eighth richest family in France had an epiphany. "All of a sudden, an idea began to dance in my head, thanks to which I'll never be bored or sad ever again: I open my eyes, then I close them, then I open them again, wide, and hey presto! I capture the image with everything - the colours! Life size! And what I hold onto is something living, that moves and feels ..." Of course, Lartigue couldn't keep everything he saw alive in his head. The world is too much. So when the boy was six, his amateur photographer father gave him his first plate camera.  And what a world Lartigue photographed: his rich family on their annual rounds to country chateaux, to Biarritz, the Cote d'Azur. Early car races, balloon flights, early wood and canvas aeroplanes. Men going off to the first world war (a war Lartigue, though a sportsman, was deemed too frail to fight). Running, jumping and standing still photos, photos of carefree days, photos of girls - especially pretty girls. But it wasn't all so innocent. In 1912, he noted in his journal that an inventor, experimenting with a parachute, had fallen to his death from the Eiffel Tower. "I wasn't there," Lartigue wrote. "What a pity for my photographs."  But mostly he photographed not death or loss or violence, but good times and ordinary times, much as the rest of us do - if his life could ever be counted as entirely ordinary. He did so methodically, obsessively. He also wrote beautifully, and his notebooks kept pace with his photography albums, in which he arranged and rearranged and montaged all his images. There is so much to read, to pore over and look at in this wonderful exhibition that one feels there, with Lartigue, at every step.  Yet Lartigue regarded himself as an old-fashioned amateur, a dilettante photographer. What he wanted, most of all, was to be a painter and through the 1920s and 30s he pursued that career with moderate success. Like Francis Picabia, another son of a wealthy family (and similarly in love with fast cars), Lartigue spent the 1930s drifting about the moneyed Riviera, designing decor for galas and balls at casinos, as well as painting portraits of the likes of Marlene Dietrich and Joan Crawford. His only major exhibition opened in Paris in June 1939. Had he been just a painter, we would have forgotten him.  It is his photography albums, over 150 of them, 14,500 pages of images, and his diaries and notes that intrigue us now. Not only because he was such an inveterate, tireless recorder of his world, but because the life he lived is itself so compelling. "I shall worry about the future later," he once said. His photography is also his autobiography. You cannot separate his art and life, which also tell the story of a century, as seen through the eyes of a pampered boy in belle epoque Paris, a life spanning two world wars, three marriages, affairs, the 1920s stock market crash (leaving him with no more than 300 francs to his name), a charmed but sometimes impecunious journey.  Photography was for most of his life a private obsession. Not until 1962, when some of Lartigue's albums were shown by an intermediary to John Szarkowski, the hugely influential curator of photography at New York's Museum of Modern Art, was serious interest in Lartigue's work kindled. Success was almost instant. Within a year he was given a show at the museum and had a feature in Life magazine. He was welcomed by fellow photographers as a long-lost brother. Richard Avedon visited to go through Lartigue's albums. He expected to stay an afternoon. It took him two weeks.  Lartigue has been compared to Marcel Proust, but he has as much in common with those photographers whose own diaristic and autobiographical approach - Robert Frank, Nobuyoshi Araki, Nan Goldin, Wolfgang Tillmans, Juergen Teller and a host of others - has become central to current artistic practice. If Lartigue's work also hinges on what has come to be called the "decisive moment" in photography, he accumulated so many such moments that what we end up with is less a collection of images, more a movie or an epic pictorial novel.  The American photographer William Egglestone has said that he thinks of his own photos as "part of some novel I'm doing". Lartigue's novel might be Peter Pan. At 17 he wrote: "Now and then, I feel full of sadness about growing up. I would like to be able to stay as I am (I feel so full of happiness, youth, and confidence). Sometimes I would even like to be a little boy still." In his 60s, he said: "I am nervous, restless, I want to see everything, store everything up, lose nothing, increase tenfold all my strength and time each day." And that's what he did, right up to his death in 1986, aged 92.  · Jacques Henri Lartigue: Photographs 1901-1986 is at the Hayward Gallery, London SE1, until September 5. Details: 08703 800 400.
A pampered childhood, two world wars, fast cars, countless women, bankruptcy ... Lartigue's photographs capture every moment of his compelling life.