Soosaithasan stood as the Tamil United Liberation Front's candidate in Mannar at the 1977 parliamentary election. He won the election and entered Parliament. He served as chairman of the Public Accounts Committee. Soosaithasan and all other TULF MPs boycotted Parliament from the middle of 1983 for a number of reasons: they were under pressure from Sri Lankan Tamil militants not to stay in Parliament beyond their normal six-year term; the Sixth Amendment to the Constitution of Sri Lanka required them to swear an oath unconditionally renouncing support for a separate state; and the Black July riots in which up to 3,000 Tamils were killed by Sinhalese mobs. After three months of absence, Soosaithasan forfeited his seat in Parliament on 22 October 1983.