McCartney took part in the civil rights march in Derry on 30 January 1972, an event widely known as Bloody Sunday. On 12 January 1979 at Belfast Crown Court McCartney and another man, Eamonn MacDermott, were convicted of the murder of Detective Constable Patrick McNulty of the Royal Ulster Constabulary, who was shot several times outside a garage in Derry on 27 January 1977. McCartney was also convicted of and the murder of businessman Jeffery Agate in February 1977, and was sentenced to life imprisonment.
These convictions were overturned in 2007.
McCartney was involved in the blanket and dirty protests, then took part in the 1980 hunger strike, along with fellow Ireland Republican Army members Brendan Hughes, Tommy McKearney, Tom McFeely, Sean McKenna, Leo Green, and Irish National Liberation Army member John Nixon. McCartney spent 53 days on hunger strike, from 27 October to 18 December.
From 1989 to 1991 he was Officer Commanding of the Ireland Republican Army prisoners in the H Blocks, and was released in 1994. McCartney was arrested on 4 April 2002 following a breach of security at Belfast"s police headquarters, but released without charge the next day.
Later that year on 5 September McCartney was the first former Ireland Republican Army member to appear before the Bloody Sunday Inquiry, and encouraged anyone with information, including paramilitaries, to come forward.
On 15 February 2007 McCartney and MacDermott had their murder convictions quashed by the Court of Appeal, following an investigation by the Criminal Cases Review Commission in 2002. Notwithstanding the normal expectation of compensation for a wrongful conviction and wrongful imprisonment, the Secretary of State for Northern Ireland declined to compensate McCartney and McDermott on the grounds that they had not proven themselves innocent. The decision was appealed to the Supreme Court of the United Kingdom which in May 2011 found in favour of the applicants, opening the way for a substantial compensation claim from both for their prison terms of 15 and 17 years.
Provisional Irish Republican Army. 2nd Northern Ireland Assembly. 3rd Northern Ireland Assembly.
4th Northern Ireland Assembly]
Since his release he has been active with ex-prisoners" groups Tar Abhaile and Coiste na n-Íarchimí, and was the first member of Sinn Féin to have their own voice heard on television after the lifting of the broadcasting ban in 1994.
McCartney has been the Member of the Legislative Assembly for Foyle since 15 July 2004, when he replaced Mary Nelis.