Mark initially had supporting roles in several British television series, including The Human Jungle and Danger Man.
In 1964, at the age of six, Lester was cast in Robert Dhery's film Allez France! (English title The Counterfeit Constable) with Diana Dors (who appeared in the 1948 film version of Oliver Twist). He played a small part as the second schoolboy in Fahrenheit 451.
In 1967, at the age of eight, Mark was cast in the title role in the film version of Lionel Bart's musical Oliver!. The multiple Academy Award-winning adaptation of Charles Dickens' novel co-starred Jack Wild, Ron Moody, Shani Wallis and Oliver Reed, and was directed by Sir Carol Reed. Since Lester could not sing, all of his singing in the movie was dubbed by Kathe Green, daughter of the film's music arranger Johnny Green. Lester was good friends with Wild throughout the making of the film and their friendship continued after production with Lester describing Wild as a "long lost brother". When Wild was nominated for an Academy Award for the film, they were flown together to the USA for the ceremony.
These two child actors later reunited for Melody (1971), which depicted British schoolchildren in love. Tracy Hyde played the role of Melody in the film, which used music from the Bee Gees and Crosby, Stills, Nash & Young.
In 1969, Lester received critical acclaim for his portrayal of a dysfunctional and withdrawn only child in Run Wild, Run Free, starring opposite John Mills, and then as a disturbed child in the first regular episode of Then Came Bronson. Lester's acting roles peaked as he starred in Eyewitness (1970), with Susan George, Night Hair Child with Britt Ekland, Whoever Slew Auntie Roo?, with Shelley Winters, Melody and a film version of Black Beauty (all 1971). After this period, his acting roles in the UK would begin to wane as he found good roles harder to come by. He extended his range with roles in a series of films in Italy including Redneck (1972) with Telly Savalas and the Western Scalawag (1973) with Kirk Douglas. The final film of his Italian-based career was in the costume drama La Prima volta sull'erba (English title The First Time on the Grass, 1974), which was nominated for the Golden Bear prize at the 25th Berlin International Film Festival. Lester wrapped up his British film career with the lead role in the all-star film Crossed Swords aka The Prince and Pauper (1977), starring Raquel Welch, Charlton Heston, Rex Harrison, George C. Scott, and Oliver Reed, who had played Bill Sikes in Oliver!.
Lester reprised his acting career in 2011 as King Harold II in 1066 currently in pre-production
Despite training to be a sociologist, Moody began appearing in theatrical shows and later decided to become a professional actor.
Moody has worked in a variety of genres, but is perhaps best known for his starring role as Fagin in Lionel Bart's stage and film musical Oliver! based on Oliver Twist by Charles Dickens. He created the role in the original West End production in 1960, and reprised it in the 1984 Broadway revival, garnering a Tony Award nomination for Best Actor in a Musical. For his performance in the film version (1968), he received the Golden Globe Award for Best Actor (Musical/Comedy), the Best Actor award at the 6th Moscow International Film Festival and an Academy Award nomination for Best Actor.
He appeared in several children's television series, including The Animals of Farthing Wood, Noah's Island, Telebugs, Into the Labyrinth, and the Discworld series. Among his better known roles is that of Prime Minister Rupert Mountjoy in the comedy The Mouse on the Moon (1963), alongside Margaret Rutherford, with whom he appeared again the following year in Murder Most Foul (1964). He played French entertainer and mime artist The Great Orlando in the 1963 Cliff Richard film Summer Holiday. He acted again with former Oliver! co-star Jack Wild in Flight of the Doves.
In 1969, Moody was offered, but declined, the lead role in Doctor Who, following the departure of Patrick Troughton from the part. He later told many people (including Doctor Who companion Elisabeth Sladen) that declining the role was a decision he subsequently regretted. He played Edwin Caldecott, an old nemesis of Jim Branning in EastEnders. He played Ippolit Vorobyaninov alongside Frank Langella (as Ostap Bender) in Mel Brooks' version of The Twelve Chairs (1970). In 2003, he starred in the black comedy Paradise Grove alongside Rula Lenska. In 2005, he acted in the Big Finish Productions Doctor Who audio play Other Lives, playing the Duke of Wellington.
In 2004, the British ITV1 nostalgia series After They Were Famous hosted a documentary of the surviving cast of the film Oliver!. Several of the film's musical numbers were reenacted. Moody, then 80 but still spry, and Jack Wild (seriously ill with oral cancer at the time) recreated their dance from the closing credits of the film.
Moody appeared in an episode of BBC1's Casualty (aired on 30 January 2010) as a Scottish patient who had served with the Black Watch during the Second World War.
On 30 June 2010, Moody appeared on stage at the end of a performance of Cameron Mackintosh's revival of Oliver! and made a humorous speech about the show's 50th anniversary. He then reprised the "Pick a Pocket or Two" number with the cast.
At the start of 2010, Moody began taking part in Memorabilia, where he greeted fans along with signing autographs.