Thornton was born on 15 January 1921 in Dulwich, London, England. He studied at Alleyn's School and at the London School of Dramatic Art. Thornton was married to Beryl Jane Margaret Evans from 1945 until his death in 2013. They had a daughter, Jane Thornton. He died on 16 March 2013 in his home in Barnes, London, England from natural causes, aged 92.