In 1957, Marks married sculptor and artist Averil Sherrard and had two children. Marks was known to have been an atheist and a humanist who was opposed to religion. Along with his wife, he campaigned for various causes including saving a park in Guildford, Surrey, where they lived, from developers. His brother John Marks was also a doctor, and the chair of the British Medical Association. 
Marks died on 6 November 2023, at the age of 93.