[3] Sampanthan joined the Illankai Tamil Arasu Kachchi (Federal Party) in 1956.

[11] Sampanthan 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.

[12] Sampanthan served as joint treasurer, vice president and general secretary of TULF.

Sampanthan contested the 2001 parliamentary election as one of the TNA's candidates in Trincomalee District.

[24] This caused the members of TULF who wished to remain with the TNA, led by Sampanthan, to resurrect the Illankai Tamil Arasu Kachchi political party.

[28][29][30][31][32] During the 2015 Sri Lankan presidential election, the TNA, under his leadership, decided to back Maithripala Sirisena as the common opposition candidate.

[33] Sirisena went on to win the election, succeeding incumbent president Mahinda Rajapaksa.

Following the 2015 Sri Lankan parliamentary election, the Speaker of the Parliament recognised Sampanthan as Leader of the Opposition on 3 September 2015.