It couldn't last, of course. In the Seventies he outgrew his typecast image, and his acting career bombed. He became tabloid fodder, partying with Oliver Reed, blowing a sizeable amount of his film fortune on wild living. Then he disappeared from sight.
Until now. Here he is in Stroud, Gloucestershire, now aged 35 and beginning a new career as an osteopath, treating bad backs and sports injuries. He shares a comfortable three-storey Georgian house in Cheltenham with his wife, Jane, and daughter, Lucinda, two. In the basement he proudly shows off his new clinic, complete with adjustable couch and posters of spinal columns.
He hasn't really changed that much. The mop of fair hair is now trimmed and auburn, and his once-slight figure is now thicker-set. But his face still has that waif-like quality once said to bring out the maternal instincts in any woman.
Despite his transformation, I still half expect him to simper and break into song with 'Who will buy this wonderful morning?' Instead he offers me a cup of tea.
He talks matter-of-factly about his child star days - the grand car turning up at home to take him to Shepperton studios, jetting around the world, playing the part of little angel for the press and fans. It's as if it were all some big childhood ritual, something you just get on with.
For him that part was easy. It was adulthood he found more difficult. He says: 'I blew about pounds 70,000 on stupid things - a very expensive car which got written off, and nightclubs. I'd always pick up the bill. It's very easy to spend a lot of money in a short space of time, going out. I put it down to an irresponsible 18-year-old, which I think I was at the time, being given a lot of money.
'I took drugs at parties and things. But no more than anybody else at that time. I had a puff on a joint once in a while, but that's about it, really. I had one or two harder drugs occasionally, but I never really liked them. I probably would have done exactly the same again. I can't say I regretted it - I don't. I never did.'
Mark Lester was born in Oxford in 1958. His parents, Mike and Rita, were both models and worked as extras in films and television. Mark, the older of their two children, toddled easily in their footsteps, and on to stage school aged five. Soon his pretty face had him in constant demand for commercials.
Then came parts in films like Space Flight 1C-1 and Fahrenheit 451. He laughs at the mention of the latter. 'I was in it for a split second. There's a scene with Julie Christie standing on a bridge. I walk past her. It was hardly a major role.'
In 1967, talent scouts were out looking for a boy to play Oliver Twist. They auditioned thousands, but Mark was recommended to the director, Carol Reed. 'I wasn't asked to sing, I wasn't asked to dance. No, they just wanted someone who looked right for the part. I just happened to fit.'
It seemed just another filming schedule but this time Mark was treated like royalty. 'I remember sneezing one day and they came and whisked me off to Harley Street. This consultant came down and diagnosed some kind of sinus problem that I never knew I had. Anyway he gave me antibiotics and I went bright red because I was allergic to them.'
Under his parents' watchful eye, he travelled the world to promote Oliver] 'America, Japan, Hong Kong - places I'd never been to before. I was picked up at the airport, by-passing customs - that was quite good fun, I suppose. Sometimes it got a bit boring - especially in America, where little old ladies with their blue-rinsed hair would come up and grab you by the cheeks and go 'aren't you cute?' My father used to say, 'well that's the price of fame'.
'I don't think I was a monstrous child. I don't think I was ever allowed to be. If I ever got out of hand I'd always get a clip round the ear. I wasn't allowed to become precocious.'
After Oliver] he continued with a series of films including Run Wild, Run Free, Black Beauty and The Prince and the Pauper. There were also some very curious films. One was The Graduation Trip, made in Japan. 'I spoke in English and they dubbed it into Japanese. It was a ridiculous thing about a child who'd gone over to Japan. His parents were in the diplomatic field, and he got involved with diamond smugglers.'
As a result, on a visit there at the age of 14, Mark was mobbed by schoolgirls in scenes of Beatle-esque hysteria. He recalls: 'I remember getting into the car and them rocking it. I thought the thing was going to go over on to its roof. And I remember running down a street once, all these Japanese kids were following me when I dropped my sunglasses on the floor. I bent down to pick them up and saw them all bearing down on me. I just shouted 'STOP]' and all these kids just stopped 10 yards behind me, and crashed into one another. In this country I could go down to the shops and to the cinema and no one even batted an eyelid.'
In his day he was reckoned to be the world's highest-paid child actor, estimated in one interview to be earning pounds 100,000 a year. He was allowed five shillings (25p) pocket money a week. Asked what he would do with all the money in the world, he replied that he'd like a spaceship. And when he grew up, he intended to be an astronaut.
At 18, he had access to some of the fortune he had amassed. And instead of a spaceship, he bought a Ferrari. He also bought a house in Belgravia and went to parties, to nightclubs, to restaurants with friends, often picking up the tab. He dabbled with drugs.
The inevitable headlines followed: 'The child star who wanted more and more', 'Oliver's sad Twist', 'Prince Mark is a Pauper'.
'The Ferrari went. I actually started driving a Mini. I suppose I went from living in Belgravia to living in Knightsbridge. It was hell,' he says with irony. 'I had to walk further to Harrods food hall.' His new-found poverty was cushioned by the fact that his father had withheld two- thirds of the film's earnings.
Mark insists he never expected his acting career to continue into adulthood: 'Child actors going on to become adult actors never really works, apart from a few. Jodie Foster was the exception.'
For a time he waited on tables at his father's Covent Garden restaurant, but soon grew bored. 'No one really bothered me. I think people expected me to look like I did in Oliver. People used to come up and ask for autographs occasionally - usually giggling girls. I used to appear in these teen magazines from time to time.'
Then he tried his hand at computers, but that went nowhere. He teamed up with a friend and had a bash at some mobile catering. But it failed when they tried to introduce American yoghurt ice cream to Britain during one of the coldest, wettest summers in memory.
In his twenties, Mark became a karate black belt. Through that he grew interested in sports injuries, and from there osteopathy. But the British School of Osteopathy shook their heads - they required A-levels and Mark didn't have any, having left school at 16.
Going back to school was a humbling experience. 'I was 28 - the oldest student at the college. There were kids doing three A-levels. These kids were in their teens, and they learn really fast. Of course I had to slog away a lot harder.'
Nevertheless he passed his chemistry and biology A-levels and began his four-year course at the British School of Osteopathy in London. There he met Jane, a former radiographer. They married more than two years ago. Now the couple both have one more exam left before they qualify, then they will run the osteopathy practice together in Cheltenham and nearby Stroud.
'They do put my films on TV from time to time. I don't go out of my way to watch them. But I'm now made to tape them for my daughter so that when she's old enough she can say 'Hey, that's Daddy.'
'On the osteopathy course I get to work in a clinic. Occasionally we have people from the theatre coming in. I get, 'Weren't you once that actor?' - 'Yes I was' - 'That's funny, isn't it?' - 'Ha, ha, ha' - then I treat them.
'At least now I can wrench their necks off if they get out of order,' he says, with just a hint of that angelic smile.