MacKenzie wrote many notable Hollywood films, including: The Private Lives of Elizabeth and Essex (1939), They Died with Their Boots On (1941), Ivanhoe (1952), and The Ten Commandments (1956).
[7] In October 1943, RKO announced they would make a film from his original story, The Spanish Main.
[9] The project was postponed due to the Palestine Cris[clarification needed] and instead MacKenzie was assigned to do a biopic on Ludwig II for producer Robert Fellows.
[15] In July 1957, he was writing Peter and Catherine about Russia in the 18th century for Ross Hunter at Universal.
[16] In late 1958, MacKenzie was reported to be working on a biopic of William the Conqueror for Evyan Perfumes.