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 on 30 January 1972, when thirteen civilians were shot and killed in the city by British soldiers of the 1st Battalion, Parachute Regiment during a civil rights march, with a fourteenth victim dying four months later.

[14][15] During the 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.

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.

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.

[18] After his release, and another conviction in the Republic of Ireland for IRA membership in 1974,[19] he became increasingly prominent in Sinn Féin, the political wing of the republican movement.

[21] 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.

It accused him of continuing involvement in IRA activity, of attending an interrogation and of encouraging Frank Hegarty, a British informer, to return to Derry from a safe house in England.

[23] In 2005, Michael McDowell, the Irish Tánaiste, stated McGuinness, along with Gerry Adams and Martin Ferris, were members of the seven-man IRA Army Council.

Experienced Troubles journalist Peter Taylor presented further apparent evidence of McGuinness's role in the IRA in his documentary Age of Terror, shown in April 2008.

At the time of his death, former US President Bill Clinton noted that McGuinness was the one who personally oversaw the agreement's arms decommissioning phase.

Having contested Foyle unsuccessfully at the 1983, 1987 and 1992 Westminster elections,[29][30][31] he became MP for Mid Ulster in 1997 and after the agreement was concluded, was returned as a member of the Assembly for the same constituency.

[33] In the weeks following the St Andrews Agreement, the four biggest parties—the DUP, Sinn Féin, the Ulster Unionist Party and the SDLP—indicated their choice of ministries in the Executive and nominated members to fill them.

McGuinness had earned endorsements from Colm Meaney, Roma Downey, and Fionnula Flanagan among others; and had performed well considering Sinn Féin's position in the Republic at that time.

"[43] To do this, he was made Steward of the Manor of Northstead on 2 January 2013 by Chancellor George Osborne, making him an employee of the Crown and thus ineligible for membership of the House of Commons.

The incentive would cost the Northern Ireland Executive £480m over 20 years, and was marred by allegations of fraud, which were not acknowledged or acted upon by Foster or the DUP (McGuinness' and Sinn Féin's partner in government).

[50] McGuinness resigned the following day; in his statements to the press, he said "Today is the right time to call a halt to the DUP's arrogance", and said that Foster had a "clear conflict of interest" in the affair.

[51] Sinn Féin refused to nominate a successor to McGuinness before 16 January, resulting in the Secretary of State for Northern Ireland, James Brokenshire, calling for new elections scheduled for 2 March.

[61] In March 2019, McGuinness was posthumously awarded a certificate of honour by mayor of San Francisco London Breed for his "courageous service in the military."

[63] In December 2016, McGuinness was advised against making a planned visit to China on medical grounds,[53] initially announcing that this was due to "unforeseen personal circumstances.

McGuinness complained that the Times had breached his privacy and that the paper had inaccurately reported the condition as genetic, thereby causing distress to his family.