He joined the Provisional IRA in the 1970s, and negotiated along with Gerry Adams with the Secretary of State for Northern Ireland, Willie Whitlaw, in 1972. He was convicted by a Republic of Ireland court in 1973, after being caught with a car with 250lb of explosives and 5000 rounds of ammunition. He was sentenced to 6 months, and refused to acknowledge the court.
After his release, and another conviction in Ireland - this time for being a member of the PIRA, he became increasingly prominent in Sinn Fein, widely seen as the political wing of the IRA. He was elected to a short-lived assembly at Stormont in 1982, and was later banned from entering Great Britain under the Prevention of Terrorism Act.
He had become Sinn Fein's chief negotiator in the time leading to the Good Friday Agreement. He became MP for Mid Ulster in 1997, and after the Agreement was concluded, was returned as a member of the Assembly, and nominated by his party for a ministerial position in the power-sharing executive. He was re-elected to the Westminster Parliament in 2001, but along with the rest of his party has refused to take his seat, because it would mean swearing an oath of alliegance to the Queen.
In November 2003 he confirmed to the Bloody Sunday Inquiry that he had been leader of the IRA in Derry in 1972, at the time of Bloody Sunday. He further stated he left the IRA in the early 1970s - a claim which is disputed by Unionist politicians, who give a 1982 for him stepping down as Chief of Staff, and sometimes say he is still on the Army Council.