Born in Strathroy, Ontario, he graduated from the University of Western Ontario and later moved to Boston, Massachusetts to perform on the stage with the Boston Repertory Theater. After the company folded following the stock market crash of 1929, Knox returned to London, Ontario where for the next two years he worked as a reporter for The London Advertiser before moving to London, England where, during the 1930s he appeared in several films. 

He starred opposite Jessica Tandy in the 1940 Broadway production of Jupiter Laughs and in 1944, he was chosen by Darryl F. Zanuck to star in Wilson, the biographical film about US President Woodrow Wilson, for which he won a Golden Globe Award and was nominated for the Academy Award for Best Actor. However, during the McCarthy Era, he was blacklisted by the Hollywood film studio bosses and he returned to England. 

He had major roles in The Sea Wolf, Over 21, Sister Kenny, The Vikings, Europa '51, None Shall Escape, and Nicholas and Alexandra, as well as supporting roles, late in his career, in Joshua Then and Now and Tinker, Tailor, Soldier, Spy. He also wrote six adventure novels: Bride of Quietness(1933), Night of the White Bear (1971), The Enemy I Kill (1972), Raider's Moon, The Kidnapped Surgeon and Totem Dream. He also wrote plays and at least three detective novels under a pseudonym prior to 1945.

He was married to American actress Doris Nolan (1916–1998) from 1943 until his death in 1995. They had a son Andrew Knox who became an actor. He died in Berwick-upon-Tweed, Northumberland from bone cancer. 

Source: Wikipedia