Goonatillake received his medical education at the Colombo Medical School, qualifying MB BS Ceylon in 1966 and later gained the MS at that university. From 1949 to 1957 he held posts in hospitals in Ceylon and was a registrar at Colombo General Hospital for three years before coming to Britain in 1954. He held house positions at St Peter's Hospital, Chertsey, and the King Edward Memorial Hospital, London, before gaining his Fellowship and returning to Ceylon. He was visiting medical officer at the General Hospital, Kalutara, the General Hospital, Galle, and the Colombo North Hospital, Colombo, from 1960 to 1974, when he emigrated to Australia and practised thereafter as a general practitioner at Footscray, Victoria. He died in 1994 before 28 July when his diploma was returned to the College.