Being a political activist, Professor Fonseka was a prominent member of the Trotskyist Lanka Sama Samaja Party (LSSP). He was a member of the party’s central committee and politburo and lead its branch in Kotte.He was a vocal critic of private medical education and campaigned against the NCMC in the early 1980s.Fonseka was also a lyricist and composer and has produced a number of albums including Carlochita Gee (1992) and Raththaran Duwe (2006).