Samuel was born in Westminster, London, to the legendary screenwriter Brian Clemens and his wife Janet Clemens, and is related to Mark Twain, being his great, great great nephew, sharing the same name as the revered writer. His father wrote and produced The Avengers (1961), The New Avengers (1976), The Professionals (1977), Thriller (1973) to name but a few.

Clemens went to school at Bedford Modern School from 1989-98 then went on to drama school. He trained at the prestigious 'The Drama Centre London' and graduated in 2001. His first acting job was a commercial for 'Nissan' but he didn't make final cut. His first break was given to him by David Drury in Family (2003) opposite Martin Kemp.