In 1943 he married Muriel Rambukpotha (1919-1978), the daughter of Albert Rambukpotha Rate Mahatmaya. They had one child, a daughter.[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. Mediwake.[6] Tennekoon ran again at the 4th parliamentary election, held on 19 March 1960, winning the seat narrowly by 438 votes.[7] The election results however left neither of Ceylon's two major parties with a majority, with the result being the calling of another election. He was subsequently re-elected at the 5th parliamentary election held on 20 July 1960. Again it was a tight contest with Tennekoon receiving 4,833 votes (52.5% of the total vote) and 540 votes ahead of the Sri Lanka Freedom Party candidate, H. M. Navaratne.[8] He did not contest the next parliamentary election in 1965.
Tennekoon went on to serve on a number of boards, including as the chairman of the Agricultural and Industrial Credit Corporation of Ceylon.
^Members of the Legislatures of Ceylon, 1931-1972. National State Assembly Library. 1972. p. 159.
^ abMadugalle, Sunil J. (2005). Four Kandyan Families: From the Maha Disawani of Matale, Uva, and Dumbara Pasrata : a Genealogical Study Within the Historical Background and the Geographical Spread of the Kandyan Kingdom (1600-1815) 1600-2000. Vijitha Yapa Publications. p. 155. ISBN9789551266004.