Simpson was posted to the 2nd Battalion, Royal Australian Regiment in January 1954, where he was promoted to sergeant in 1955, serving in Malaya from October 1955 for the next two years. He was subsequently posted to 1st Special Air Service Company (SAS) in November 1957 and served with that unit until selected as one of the initial group members of the Australian Army Training Team Vietnam (AATTV) sent to assist South Vietnamese forces in July 1962. A year later, he returned to the SAS in Australia and served there for the next twelve months.