[1][2] After completing his secondary education at Nalanda College, Colombo,[3] he joined the Sri Lanka Police in 1972 as an acting Sub Inspector.
Mendis held numerous appointments as OIC of police stations in the north and east of the island.
In 1977, he was transferred to the Criminal Investigation Department and served as a detective, reaching the rank of Senior Superintendent of Police and Deputy Director CID.
[4] He was recalled from retirement, by President Sirisena who appointed him as Chief of National Intelligence (CNI) in July 2015.
He resigned citing health reasons, on 8 June 2019, days after giving testimony that the Parliamentary select committee to inquire into the 2019 Sri Lanka Easter bombings which had outraged President Sirisena.