An only child, Challis was born in Bristol but brought up in South East London. His father James Alec, from Sheffield, was a Civil Servant. Challis attended the state boarding Ottershaw School near Woking, Surrey. Upon leaving school he worked as a trainee estate agent before becoming a professional actor. An early television role, in 1969, was in the controversial gangster drama Big Breadwinner Hog, and between 1971 and 1975 he made regular appearances in Z-Cars as Sergeant Culshaw.