 In 1950, at the age of 18, Themis was elected the Vice President of the All Ceylon Post and Telecommunications Union and the following year was elected as its Secretary.At the 3rd parliamentary election held in April 1956 was elected as the third member from Colombo Central, a multi-member constituency, beating Dr. M. C. M. Kaleel of the United National Party by 46 votes into fourth place.On 7 July 1959 he resigned from the Sri Lanka Freedom Party and left the government, along with Philip Gunawardena to form Mahajana Eksath Peramuna (People's United Front).