[3] He then joined an apprentice course in architecture run by Peradeniya University architect, Shirley de Alwis, in 1945.
[6] He was the first Asian to win the Wunderlich Annual Prize given by the school in recognition of general excellence by students.
[3] In 1958, Tennekoon returned to Sri Lanka and was appointed assistant architect in the Public Works Department.
During his tenure at the Public Works department he was responsible for designing the National Library, Colombo (1976); Bandaranaike Samadhi, Horogolla; Prime Minister D. S. Senanayake's memorial, Colombo; Supreme Court Complex in Hulftsdorp (1978); Siyane Teacher Training College; teaching hospital complex at the Colombo South Hospital, Kalubowila; Kollupitiya police station; and the Department of National Archives.
Tennekoon is credited with giving architecture a more central role in a culture of public commissions that were heavily centred in engineering, and using perspectival drawings to convey the importance of aesthetic aspects of buildings.