In 1963 he co-founded the Republican Labour Party, and was Westminster MP for West Belfast from 1966 to 1983. Fitt was batoned while leading a civil rights march in Derry in 1968 and requested intervention by British troops.
In 1970 he became the founding leader of the Social Democratic and Labour Party (SDLP). He was elected for North Belfast to the Northern Ireland Assembly in 1973 and was deputy chief executive of the power-sharing executive. In 1980 he denounced Republican prison protests. Fitt became a life peer in 1983, having lost his Westminster and Belfast Corporation seats.