St. Thomas' College was founded on 9 March 1844 by the Christian Missionary Society of England. A main focus of the missionary bodies in Sri Lanka during the early period of
British rule in Ceylon was providing English education. As a result, St. Thomas' School later became a secondary school in 1914, as St. Thomas' College was established in a bungalow in the village of Wellamadama, the current location of the
University of Ruhuna. The school was founded by one of the early
Anglican missionaries, Rev. Fr. N. J. Ondatjee, in 1844 with several students and three teachers. Odantjee was later succeeded by others, including
Kumaratunga Munidasa, who taught the
Sinhala language and literature. In 1960, St. Thomas' College came under the control of the Government after a long period of missionary control. K. B. Jayasuriya became the first principal under the
government administration. J. E. M. Fernando, K. B. Jayasuriya and E. A. de L. W. Samarasinghe reactivated the college's old boys (i.e., alumni) association, which was defunct after its inauguration by S. J. Gunasekeram in 1934. B. D. Jayasekera designed the college flag and the crest in the early 20th century. A Buddhist shrine room was constructed at the college premises by the Old Boys Association in 1999. It was inaugurated by
Madihe Pannaseeha Thero, a
Buddhist priest and former student of the college. ==Past principals==