After recovering from the disease he moved to London, wanting to become a director. He could not find any suitable courses and enrolled in the London Academy of Music and Dramatic Art (LAMDA) to learn acting.
In the 1950s he had a number of stage roles. He made his film debut in 1958 in the film Alive and Kicking. But his first star turn was in the 1963 film This Sporting Life, as a bitter young coal miner who becomes an acclaimed rugby player.
He appeared as King Arthur in the film adaptation of Camelot (in which he was cast despite his limited singing range), and proceeded to appear on stage in that role for years. He recorded an album, including the 7-minute hit song "MacArthur Park" (which Harris mispronounced as "MacArthur's Park"); that song reached #2 on the United States Billboard magazine pop chart in the summer of 1968.
In his late career, he acted in the Oscar-winning films Unforgiven and Gladiator (in the latter as Marcus Aurelius). He gained a new generation of fans as Albus Dumbledore in the first two Harry Potter films.
Harris died of Hodgkin's disease in 2002.