Provisional IRA activity
McGuinness acknowledged that he was a former IRA member, but stated that he left the IRA in 1974.
 He originally joined the
Official IRA, unaware of the split at the December 1969 Army Convention, switching to the
Provisional IRA soon after. By the start of 1972, at the age of 21, he was second-in-command of the IRA in
Derry, a position he held at the time of
Bloody Sunday, when 14 civil rights protesters were killed in the city by soldiers of the
1st Battalion, Parachute Regiment.
Saville Inquiry into the events of that day,
Paddy Ward stated he had been the leader of the
Fianna, the youth wing of the IRA at the time of Bloody Sunday. He said that McGuinness and an anonymous IRA member gave him bomb parts that morning. He said that his organisation intended to attack city centre premises in Derry on the same day. In response, McGuinness said the statements were "fantasy", while Gearóid Ó hEára (formerly Gerry O'Hara), a Derry
Sinn Féin councillor, stated that he and not Ward was the Fianna leader at the time.
The inquiry concluded that, although McGuinness was "engaged in paramilitary activity" at the time of Bloody Sunday and had probably been armed with a
Thompson submachine gun, there was insufficient evidence to make any finding other than they were "sure that he did not engage in any activity that provided any of the soldiers with any justification for opening fire".
McGuinness negotiated alongside
Gerry Adams with the
Secretary of State for Northern Ireland,
Willie Whitelaw, in 1972. In 1973, he was convicted by the
Republic of Ireland's
Special Criminal Court, after being arrested near a car containing 250 pounds (110 kg) of explosives and nearly 5,000 rounds of ammunition. He refused to recognise the court, and was sentenced to six months' imprisonment. In court, he declared his membership of the Provisional IRA without equivocation: "We have fought against the killing of our people... I am a member of
Óglaigh na hÉireann and very, very proud of it".
After his release, and another conviction in the Republic of Ireland for IRA membership, he became increasingly prominent in Sinn Féin, the political wing of the
republican movement. He was in indirect contact with British intelligence during the
1981 hunger strikes, and again in the early 1990s.
 He was elected to the
Northern Ireland Assembly at
Stormont in 1982, representing
Londonderry. He was the second candidate elected after
John Hume. As with all elected members of Sinn Féin and the SDLP, he did not take up his seat.
 On 9 December 1982, McGuinness, Gerry Adams and
Danny Morrison were banned from entering Great Britain under the
Prevention of Terrorism Act by the
Home Secretary, William Whitelaw.
In August 1993, he was the subject of a two-part special by
The Cook Report, a
Central TV investigative documentary series presented by
Roger Cook. It accused him of continuing involvement in IRA activity, of attending an interrogation and of encouraging Frank Hegarty, an informer, to return to Derry from a safe house in England. Hegarty's mother Rose appeared on the programme to tell of telephone calls to McGuinness and of Hegarty's subsequent execution. McGuinness denied her account and denounced the programme saying "I have never been in the IRA. I don't have any sway over the IRA".
Michael McDowell, the Irish
Tánaiste, stated McGuinness, along with Gerry Adams and
Martin Ferris, were members of the seven-man
IRA Army Council.
 McGuinness denied this, saying he was no longer an IRA member. Experienced
Peter Taylor presented further apparent evidence of McGuinness's role in the IRA in his documentary Age of Terror, shown in April 2008.
 In his documentary, Taylor alleges that McGuinness was the head of the
IRA's Northern Command and had advance knowledge of the IRA's 1987
Enniskillen bombing, which left 11 civilians dead.