Born in Bellaghy, County Londonderry, Milne joined the Official Irish Republican Army-linked Fianna Éireann youth group soon after its formation, but the following year moved to join the Provisional IRA. He was gaoled in 1971, after explosives went off in a car in which he was travelling. He was imprisoned in the Crumlin Road Jail, but escaped in January 1973. The following year, he was arrested in the Republic of Ireland after stealing a Garda car, and was sentenced to five years in Portlaoise Prison. However, he again escaped, and remained an active paramilitary based in Northern Ireland.