His big break was the title role in The Adventures of Sir Lancelot on ITV in 1956, which for sale to the NBC network in the U.S. became the first British television series to be shot in colour. Following this, he won a role in Doctor Who as one of the Doctor's companions, science teacher Ian Chesterton, appearing in the bulk of the first two seasons of the programme.
Russell was one of the four original castmembers of Doctor Who in 1963, starring opposite William Hartnell as the Doctor, Jacqueline Hill as Barbara Wright and Carole Ann Ford as Susan Foreman. His first involvement in the series took the form of the untransmitted pilot episode, which was eventually reshot and aired as "An Unearthly Child". He eventually departed, alongside Hill, in the penultimate story of the second season, The Chase.
Four decades on from his first appearance, Russell continues his involvement with Doctor Who, having lent his voice as a narrator to several of the audio book releases of the 'lost' 1960s episodes. He has also appeared in The Game, one of the continuing Doctor Who audio stories produced by Big Finish. More recently, Russell has recorded readings of some of the Target Novelisations of Doctor Who episodes, also for CD release.
In the late 1990s Russell returned to the role of Ian for the VHS release of the story "The Crusade", of which episodes two and four are currently lost. He recorded several in-character scenes to camera, which helped to bridge the gap between the existing episodes.
Russell has also contributed to the Doctor Who DVD range, having participated in several audio commentaries and on-screen interviews since 2002.