He practiced for a number of years in Kandy before joining the Ceylon Civil Service with the Department of Lands.
[3] In 1956, encouraged by his maternal uncle, Herath Banda Rambukwella, the sitting member for Minipe, he resigned from his position as Assistant Settlement Officer and ran as the United National Party candidate, at the 3rd parliamentary election, in the Minipe electorate.
He was unsuccessful, losing by 4,648 votes to the Sri Lanka Freedom Party candidate, M. B. W.
[6] Tennekoon ran again at the 4th parliamentary election, held on 19 March 1960, winning the seat narrowly by 438 votes.
Tennekoon went on to serve on a number of boards, including as the chairman of the Agricultural and Industrial Credit Corporation of Ceylon.