After studying at the London School of Economics, Moffat became a socialite and began to make films to promote the war effort.
During World War II he filmed activities of the US Army, meeting director George Stevens, whom he soon followed to Hollywood and assisted at Paramount Pictures.
[2] By 1938, after his father moved back to America, Moffat joined the Gargoyle Club in Soho, hosting frequent parties in his father's famously decorated house, meeting Philip Toynbee and Dylan Thomas and working for Strand Films, where he helped to make documentaries for the government to promote the war effort.
[2] In 1943, after America entered World War II, Moffat enlisted in the US Army, serving as a writer in the Signal Corps, a special unit known as the "Hollywood Irregulars".
[2][4] After the war, Moffat settled in Hollywood, joining Stevens as an assistant producer at Liberty Films, which was soon purchased by Paramount Pictures.
[2] Moffat is interviewed extensively in the documentary George Stevens: A Filmmaker's Journey (1985) and on the DVDs of Shane and Giant.
[2] In 1956 he began a long on again, off again affair with then-married Lady Caroline Blackwood, fathering her daughter Ivana Lowell in 1966, after he was remarried.
[8] During the marriage, the couple moved back to London when Moffat was hired to "doctor" the screenplay for The Great Escape (1963), and partly so his wife could fulfil her responsibilities as lady-in-waiting to the Queen Mother.