McKee was born in Belfast in 1921, and joined the Irish Republican Army (IRA) in 1939. During the Second World War, the IRA carried out a number of armed actions in Northern Ireland known as the Northern Campaign. McKee was arrested and imprisoned in Crumlin Road Gaol until 1946 for his role in this campaign. In 1956, the IRA embarked on another armed campaign against the existence of Northern Ireland, known as the Border Campaign. McKee was again arrested and interned for the duration of the campaign. He was released in 1962.
Upon release, he became Officer Commanding (OC) of the IRA's Belfast Brigade. However, he resigned this position in 1963, after a dispute with other republicans after McKee acceded to a Royal Ulster Constabulary (RUC) demand that he not fly an Irish tricolour during a republican march. He was succeeded by Billy McMillen.
As the 1960s went on, McKee drifted away from the IRA. He grew very disillusioned with the organisation's increasing emphasis on socialism and reformist politics over "armed struggle". McKee was a devout Roman Catholic, who attended Mass daily. As a result, he was very uncomfortable with what he felt were "communist" ideas coming into the republican movement.
During the Northern Ireland riots of August 1969, severe rioting broke out in Belfast between Irish Catholic nationalists, Protestant loyalists, and the RUC. McKee was highly critical of the IRA's failure to defend Catholic areas during these disturbances. On 14 August 1969 McKee, Joe Cahill and a number of other republicans occupied houses at Kashmir Street in an effort to fight off the RUC and loyalists. However, being poorly armed, they failed to prevent Catholic Bombay Street and parts of Cupar Street and Kashmir Street being burned out.
In the aftermath of the riots, McKee accused Billy McMillen, the IRA's Belfast commander, and the Dublin-based IRA leadership, of having failed to provide arms, planning or manpower to defend Catholic streets. On 22 September, McKee and a number of other IRA men, arrived armed at a meeting called by McMillen and tried to oust him as head of the Belfast IRA. They did not succeed, but announced that they would no longer be taking orders from the IRA leadership in Dublin. In December of that year, the IRA split into the Provisional IRA which was composed of traditional militarists like McKee, and the Official IRA which was composed of the remnants of the pre-split Marxist leadership and their followers. McKee sided with the Provisionals and sat on the first Provisional Army Council in September 1970.
McKee became the first OC of the Provisional IRA Belfast Brigade. From the start, there was intermittent feuding between McKee's men and his former comrades in the Official IRA, as they vied for control of nationalist areas. However, the Provisionals rapidly gained the upper hand, due to their projection of themselves as the most reliable defenders of the Catholic community.
McKee himself contributed greatly to this image by an action he undertook on 27 June 1970. Rioting broke out in the Ardoyne area of north Belfast after an Orange Order parade, and three Protestants were killed in gun battles between the Provisional IRA and loyalists. In response, loyalists prepared to attack the vulnerable Catholic enclave of Short Strand in east Belfast. When McKee heard about this, he drove to Short Strand with some men and weapons and took up position at St Matthew's Church. In the ensuing five-hour gun battle, McKee was wounded and one of his men was killed, along with at least four Protestants.
On 15 April 1971 McKee, along with Proinsias MacAirt, was arrested by the British Army when found in possession of a hand gun. He was charged and convicted for possession of the weapon and imprisoned in Crumlin Road Prison, and Joe Cahill took over as OC of the Belfast Brigade.
In 1972, McKee led a hunger strike protest in an effort to win recognition of IRA prisoners as political prisoners. Republicans who were interned already had special status, but those convicted of crimes were not. When McKee was close to death, William Whitelaw conceded Special Category Status which, although not officially awarding political status, was tacit recognition of the political nature of the incarceration.
McKee was released on 4 September 1974 and resumed his position as OC of the Belfast Brigade. At this time the Provisional IRA called a ceasefire and McKee was involved, with Ruairí Ó Brádaigh in secret peace talks in Derry with the Northern Ireland Office. He was also involved in talks with Protestant clergy in Feakle, County Clare in December 1974, where he voiced his desire to end the violence.
However, in the same period, McKee authorised a number of sectarian attacks on Protestants as well as renewed attacks on rival republicans in the Official IRA. For this he was heavily criticised by a group of Provisional IRA activists grouped around Gerry Adams.
A faction led by Adams managed to get McKee voted off the IRA Army Council in 1977, effectively forcing him out of the leadership of the organisation. McKee's health suffered in this period and he did not resume his IRA activities. He joined Republican Sinn Féin after a split in Sinn Féin in 1986.
In more recent years McKee, Brendan Hughes and Tommy McKearney were critical of the Belfast Agreement and of the reformist politics of Sinn Féin.