He joined the newly formed Royal Ceylon Air Force as its fifth recruit (1005 AC2) as a Cadet Officer on 8 August 1951.

The government decided to deploy more transports instead of the Vampires, Mendis trained for Airspeed Oxfords in the flying wing.

Promoted to the rank of Group Captain, he attended the Imperial Defence College in London in 1968 and was appointed Chief of Staff in December 1969.

Though unprepared for a full-scale insurrection, with limited aircraft, under Mendis' command, the Air Force responded effectively.

Following the insurrection, military spending was drastically cut as a result of major economic problems faced by the country.