After university Kunaratnam worked at the University of Ceylon, Colombo and University of Ceylon, Peradeniya.[3] He studied at Imperial College London between October 1960 and July 1963, obtaining a DIC and a PhD degree.[3]
Kunaratnam was visiting fellow and lecturer in geophysics at the Department of Geology, University of Durham between January 1971 and September 1971.[3] He was visiting fellow at the Department of Physics, Queen Elizabeth College from October 1971 to December 1971.[3]
Kunaratnam was appointed Founder Head of the Department of Physics at the University of Sri Lanka Jaffna campus in April 1975.[4] He remained head of the department until October 1980.[4] He was visiting lecturer at the School of Physics, University of Science, Malaysia between Jan 1981 and October 1981.[3] He then returned to the University of Jaffna and resumed being Head of the Department of Physics, serving until February 1985.[4] He was visiting fellow and professor at the Department of Geophysics, University of Edinburgh between October 1988 and August 1989.[3] He was visiting fellow at the Department of Earth Sciences, Bullard Laboratories, University of Cambridge between Sept 1989 and October 1989.[3]
Kunaratnam was Head of the Department of Computer Science at the university between January 1991 and November 1993.[5] He then served as Head of the Department of Physics from March 1994 and April 1994.[4]
Kunaratnam served as dean of the Faculty of Science twice, from January 1977 to May 1978 and January 1985 to March 1988.[6] He served as vice-chancellor of the university between April 1994 and February 1997.[7] Kunaratnam retired in April 2000.[3] He served as an emeritus professor at the Department of Physics till his death.[8][9]
Kunaratnam was elected fellow of the Royal Astronomical Society in 1998 and fellow and chartered Physicist of the Institute of Physics in 1999.[3] He was awarded an honorary doctorate from the University of Jaffna in 2001.[3] He published several papers in scientific journals on magnetic anomalies in earth magnetic field.[10]
Kunaratnam died on 9 September 2015 in Colombo.[11][12]