Amirthalingam joined the newly formed Illankai Tamil Arasu Kachchi (Federal Party) in 1949 and became leader of its Youth Front.

[5][6] He stood for re-election in Vaddukoddai at the 1970 parliamentary election but was defeated by the All Ceylon Tamil Congress candidate.

Amirthalingam was delivering leaflets along with other leading Tamil politicians (M. Sivasithamparam, V. N. Navaratnam, K. P. Ratnam and K. Thurairatnam) in 1976 when they were all arrested on government orders.

All the defendants were acquitted after a famous trial at bar case in which 72 Tamil lawyers including S. J. V. Chelvanayakam and G. G. Ponnambalam acted for the defence.

[9] Amirthalingam 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 murdered by Sinhalese mobs.

Amirthalingam and his wife moved into a house on Bullers Road (Baudhaloka Mawatha) in the Cinnamon Gardens area of Colombo.

The house was shared with other leading TULF politicians (M. Sivasithamparam, V. Yogeswaran and Mavai Senathirajah) and their families.

He arranged a meeting between the Tamil Tigers and the TULF leaders at their Bullers Road residence.

On the evening of 13 July 1989 three men, Peter Aloysius Leon (Vigna), Rasiah Aravindarajah (Visu) and Sivakumar (Arivu), arrived at the residence.