Born in the Adelaide suburb of Medindie, South Australia, Derrick left school at the age of fourteen and found work in a bakery. As the Great Depression grew worse he lost his job and moved to Berri, working on a fruit farm before marrying in 1939. In July 1941, Derrick enlisted in the Second Australian Imperial Force, joining the 2/48th Battalion. He was posted to the Middle East, where he took part in the siege of Tobruk, was recommended for the Military Medal and promoted to corporal. Later, at El Alamein, Derrick was awarded the Distinguished Conduct Medal for knocking out three German machine gun posts, destroying two tanks, and capturing one hundred prisoners.