Mac Stíofáin is born John Stephenson, the son of Protestant parents. He claims Irish ancestry on his mother’s side although the validity of this is uncertain. He leaves school at sixteen, working as a labourer and converting to Catholicism. He also serves in the Royal Air Force during World War II, working as a storeman. After the war, he becomes involved and obsessed with Irish republicanism. He joins the Irish Republican Army in 1949 and helps organise an IRA unit in London.
In 1953, Stephenson leads a raid that steals rifles and mortars from a cadet school armoury in Essex. He is stopped randomly by police, arrested and sentenced to eight years in prison. He serves more than three years behind bars, using this time to learn Irish Gaelic. Released in 1956, he marries an Irish woman, moves to Dublin and changes his name to Seán Mac Stíofáin, the Gaelic form of his birth name.
Mac Stíofáin gradually ascends through the ranks of the IRA, becoming its director of intelligence. The outbreak of the Troubles in 1969 opens up divisions in the IRA over strategy and tactics. While Cathal Goulding and other leaders want to use violence carefully, Mac Stíofáin and his supporters urge open warfare with the Royal Ulster Constabulary (RUC).
In August 1969, Mac Stíofáin leads a raid on the RUC station at Crossmaglen, in defiance of IRA orders. In December, he and four others form a Provisional Army Council. This splinter group becomes the nucleus of the Provisional IRA.
Mac Stíofáin becomes the Provisional IRA’s first chief of staff. He also oversees its rearming and the escalation of its military campaign in Northern Ireland. In July 1972, he represents the Provisional IRA in secret talks with the British government in London. When these talks collapse he orders an increase in Provisional IRA operations, beginning with the mass bombing of Belfast on July 21, 1972.
Mac Stíofáin remains in charge until November 1972, when a controversial television interview leads to his arrest, imprisonment and removal from the Provisional IRA leadership. He is released the following year but is no longer prominent in the Provisional IRA. He spends the rest of the 1970s working for a Sinn Féin newspaper.
Mac Stíofáin died on May 18, 2001 in Our Lady’s Hospital in Navan, County Meath, after a long illness. He is buried in St. Mary’s Cemetery, Navan. His funeral is attended by Gerry Adams and Martin McGuinness.