Moore was born in Stockwell, London (sometimes referred to by Moore in interviews as Saint Ockwell). The son of Lillian "Lily" (née Pope), a housewife, and George Moore, a policeman, he attended Battersea Grammar School, but was evacuated to Holsworthy, Devon during World War II, where he attended Launceston College and was then educated at Dr Challoner's Grammar School.

In the early 1950s, Moore worked as a male model, appearing in print advertisements for knitwear (earning him the amusing nickname "The Big Knit"), and a wide range of other products such as toothpaste, an element that many critics have used as typifying his lightweight credentials as an actor. His earliest known television appearance was on 27 May 1950, in Drawing Room Detective, a one-off programme.

Presented by veteran BBC announcer Leslie Mitchell, it invited viewers at home to spot clues to a crime during a playlet, whose actors also included Alec Ross (first husband of Sheila Hancock) and Michael Ripper. Barring interviews, Moore has not worked for BBC television since.

Although Moore won a contract with MGM in the 1950s, the films which followed were not a success and, in his own words, "At MGM, RGM (Roger George Moore) was NBG [no bloody good]." His starring role in The Miracle, a version of the play Das Mirakel for Warner Bros., had been turned down by Dirk Bogarde.

Eventually, it was television in which Moore made his name. He was the eponymous hero in the serial Ivanhoe, a very loose adaptation of the romantic novel by Sir Walter Scott, and he also appeared in the series The Alaskans, as well as playing Beau Maverick, an English cousin of frontier gambler Bret Maverick (James Garner) in Maverick.

It was not until 1961 that worldwide fame arrived, when Lew Grade cast Moore as Simon Templar in a new adaptation of The Saint, based on the novels by Leslie Charteris. Moore said in an interview, during 1963, that he wanted to buy the rights of Leslie Charteris's character and the trademarks, but didn't have enough money.

He also joked that the role was supposed to have been meant for Sean Connery who was unavailable. The television series was made in the UK with an eye on the American market, and its success there (and in other countries) made Moore a household name. It also established his suave, quipping style which he would carry forward to James Bond.

Moore would also go on to direct several episodes of the later series, which moved into colour in 1967. The opinion has often been expressed that the monochrome episodes of the series, which were closer adaptations of Charteris's work, were superior to the colour episodes, which displayed a stronger leaning towards fantasy and were arguably trying too hard to imitate other shows of that time.

The Saint ran for six seasons and 118 episodes, making it (in a tie with The Avengers) the longest-running series of its kind on British television. However, Moore grew increasingly tired of the role, and was keen to branch out. He made two films immediately after the series had ended: Crossplot, a lightweight 'spy caper' movie, and the more challenging The Man Who Haunted Himself (1971). Directed by Basil Dearden, it gave Moore the opportunity to demonstrate a wider versatility than the role of Simon Templar had allowed, although reviews at the time were lukewarm, and both did little business at the box office.

Television lured Moore back to star, alongside Tony Curtis, in what has become another cult series, The Persuaders!. Even more light-hearted in tone than The Saint, it featured the adventures of two millionaire playboys across Europe. It was for this series that Moore was paid the then unheard-of sum of one million pounds for a single series, making him the highest paid television actor in the world. However, Lew Grade claimed in his autobiography Still Dancing, that Moore and Curtis "didn't hit it off all that well". Curtis refusing to spend more time on set than was strictly necessary, while Moore was always willing to work overtime.

The series failed in America, where it had been pre-sold to ABC but it was successful in Australia and in Europe. In Germany, where the series was aired under the name Die Zwei, it became a hit through a special funny dubbing that only barely used the original translations of the dialogs. And in Britain it was also popular, although on its premiere on the ITV network, it was beaten in the ratings by repeats of Monty Python's Flying Circus on BBC1.

When Channel 4 repeated both The Avengers and The Persuaders! in 1995, it was generally agreed that the latter, which had not been seen for many years, had not aged as well as the former. It has not been seen on any of the five main UK terrestrial channels since.

Since then, The Persuaders has enjoyed something of a renaissance both on television and DVD, with the 'rivals' Moore and Curtis reuniting to provide commentaries on the most recent issues. In France, where the series (entitled Amicalement Vôtre) had always been popular, the DVD releases accompanied a monthly magazine of the same name.

There are many apocryphal stories as to when Moore's name was first dropped as a possible candidate for the role of James Bond. Some sources, specifically Albert R. Broccoli from his autobiography When The Snow Melts, claim that Moore was considered for Dr. No, and that he was Ian Fleming's favourite for the role after apparently having seen Moore as Simon Templar in The Saint; however, the series did not begin airing in the United Kingdom until October 4, 1962 – one day before the premiere of Dr. No, although it's possible that the show began filming before or around the film.

Other sources, such as the for the special edition DVDs, claim that Moore was passed over for Bond in favour of someone who was older. As Moore is older than Sean Connery, this is probably not true. Publicly, Moore was not linked to the role of 007 until 1967, when Harry Saltzman claimed he would make a good Bond, but also displayed misgivings due to his popularity as Simon Templar. Nevertheless, Moore was finally cast as James Bond in Live and Let Die (1973).