Pathmanathan joined the University of Ceylon, Peradeniya in 1963 as an assistant lecturer in history. He was promoted to lecturer (1969), senior lecturer (1975) and associate professor (1981). He was a visiting professor at
Eastern University, Sri Lanka (2007–08) where he helped establish the Hindu Civilisation
discipline. Pathmanathan has served in various positions including vice chairman of the
University Grants Commission (1994–99) and a member of the National Education Commission (1991-2001). In 2026, Pathmanathan published *Glimpses of an Ancient Civilisation: Society and Culture in Jaffna (300 BC to AD 500)*, which examines the society and culture of early
Jaffna from 300 BC to AD 500.A review by
Jehan Perera described the work as a significant contribution to the study of Sri Lanka’s early history, particularly in its analysis of settlement patterns, cultural developments, and links with
South India. ==Works==