Fonseka was educated at Saint John"s College Panadura, Royal College, Colombo and went on to study at Downing College, Cambridge, there he gained his Master of Arts. After leaving Cambridge entered the Inns of Court and qualified as a barrister.
He was the Deputy Speaker of the State Council of Ceylon and following Ceylon"s independence from Britain her first Ambassador to Burma and first Ambassador to Japan. De Fonseka took an active part in the country"s struggle for universal suffrage and self-determination. Apron his return to Ceylon he became the principal of Sri Sumangala College, Panadura after which he went on to become the editor of The Morning Leader.
Following his journalism career he entered the State Council as member for Panadura for the first time in 1931 and again after the 1936 general election.
De Fonseka took an active part in the country"s struggle for universal suffrage and self-determination. First it was with the Donoughmore Commission and later with the Soulbury commissioners (1943 - 1945).
Susantha de Fonseka as the State Council member for Panadura, moved a motion in the State Council, by November 1944, to the effect that the Ministers "be directed to introduce immediately a Bill conferring on this country a constitution of the recognized dominion type for Free Lanka". He joined the newly formed Ceylon Naval Volunteer Force as the first Ceylonese commissioned officer, later his commission was withdrawn for stating in the State Council that the British were under-pricing Ceylon"s rubber.
In 1949 Susantha de Fonseka was appointed as Ceylon"s first High Commissioner to Burma.
He was a very popular figure in Rangoon, often making speeches in Burmese language. Ambassador de Fonseka built up such close contacts with the Burmese leadership that he was associated with Prime Minister U Na when Karen guerrillas surrendered. Thereafter Doctorate. South. Senanayake sent him on an important diplomatic assignment to Peking in 1952.
lieutenant was this assignment which laid the foundation for the rubber-rice pact between Ceylon and China.
Fonseka later became Ceylon"s first Ambassador in Japan. Susantha de Fonseka was made a Knight Commander of The Most Excellent Order of the British Empire on 12 April 1954.
Sir Susantha de Fonseka, the Father of the Free Lanka Bill is remembered as one Sri Lanka"s pioneering diplomats and for promoting constitutional reforms on the long walk to independence. In his memory the Sir Susantha De Fonseka memorial Prize for Arts is awarded annually at Royal College, Colombo.
He lost his seat in the 1947 general election to a candidate from the Lanka Sama Samaja Party, marking the end of his political career.