Mr. Castle was involved in amateur theatre at College, though after leaving, he didn't immediately pursue acting as a profession. Instead, he became a clerk in a ball-bearing manufacturing plant, a hotel waiter, a travel agent, a clerk in a petroleum cracking plant, a brassiere salesman, a mail sorter, a landscape gardener, a construction worker and a Geography teacher. Referring to that diverse series of occupations, Mr. Castle was quoted as saying "I didn't begin to think seriously about anything, including myself, for years.". His wife convinced him to again pursue acting. Then, he managed to be granted a scholarship at R.A.D.A., where he would begin to build a stage career. He understudied for the role of Shakespeare's "Henry V". However, when the principal actor took ill, he took the stage and was praised for his brilliance. This gave him the encouragement to join the New Shakespeare Company, with whom he toured the Far East for six months. He returned to make an appearance in "Saved" at the Royal Court Theatre. Then, most notably, he portrayed Malcolm in the controversial "Macbeth" starred in by Alec Guinness and Simone Signoret. His first experience in film came in a small part (as an artist, a painter) in Antonioni's "Blow Up."

John Castle was born in Croydon, England and raised in Brighton from the ages of six to eighteen with his two sisters and a younger brother. His wife Maggie Wadey is a writer, and they have one daughter, Shelley.