Cousins joined the RAF in 1961 and spent three years at Royal Air Force College Cranwell. He then had a number of operational flying tours, initially flying Lightnings in the air defence role in the UK and with RAF Germany and then Buccaneers for RAF Germany. In 1983 he became Station Commander at RAF Laarbruch, home to two RAF Squadrons flying Jaguars and Tornados.
He then held a number of staff appointments in air plans, operational requirements and operations. Following attendance at the Royal College of Defence Studies, he held a number of senior air rank positions on the Air Staff at the Ministry of Defence, in the MoD Procurement Executive, as Commandant of the Royal Air Force College Cranwell and, from 1994, as Air Officer Commanding No. 38 (Transport) Group. He was appointed Air Member for Personnel on the Air Force Board and Air Officer Commanding-in-Chief Personnel and Training Command in May 1995 and served in that role until he retired in August 1998.
