Clark was born in 1931, and grew up in Boston, Lincolnshire. Clark moved to London, and in 1951 he began work as an assistant editor at the legendary Ealing Studios. Subsequently, Clark worked as a freelance assistant editor on two films directed by Stanley Donen and edited by Jack Harris. When Harris declined the opportunity to work on Donen's subsequent film, Surprise Package (1960), Donen gave Clark the job. As Clark later wrote,
It was a fairly bad movie and probably would have finished my career before it had started, but luckily Stanley got another film, The Grass Is Greener (1960), which he also asked me to edit. The cutting of the two films overlapped, which was great training for me. Soon after, I was asked to cut The Innocents (1961) starring Deborah Kerr, which has since become a classic story-driven, supernatural horror film. It was very hard to cut, but that film really put me on the map.
Responding to a question about the major influences on his editing, Clark said
Looking back over many years, the American cinema of the 40s was very important to me, along with Hitchcock films and early British comedies with actors like Will Hay and George Formby. Music has also played a major role; it influences the rhythm of my editing. The pacing of a film and its dialogue have a lot to do with music, and the act of going from one shot to another has always fascinated me - when and how you do it, the reason for an edit.
Clark now lives in Kensington with his wife Laurence Méry-Clark, who is also a film and television editor. They married in 1961 and have three children. In 2011 Clark's autobiography Dream Repairman: Adventures in Film Editing was published, receiving warm reviews from The Guardian and The Observer.