Knox portrayed Control in the 1979 BBC miniseries adaptation of John le Carre's Tinker Tailor Soldier Spy.

Canadian novelist Robertson Davies described his performance thus: "To this role he brought a dignity which did much to heighten the effect of the famous court-scene which makes up the third act...".

However, during the McCarthy Era, his liberal views and work with the Committee for the First Amendment hurt his career, but he was not blacklisted,[3] and he returned to Britain.

He wrote several adventure novels: Bride of Quietness (1933), Night of the White Bear (1971), The Enemy I Kill (1972; republished as Totem Dream in 1973), Raider's Moon, and The Kidnapped Surgeon.

They had a son Andrew Joseph Knox (born 1947; died by suicide in 1987[citation needed]) who became an actor and appeared in Doctor on the Go, and who was married to Imogen Hassall.