McElwaine was an active member of the IRA, who became Officer Commanding of the IRA in County Fermanagh by the age of 19. On 5 February 1980, McElwaine killed off-duty Ulster Defence Regiment (UDR) corporal Aubrey Abercrombie as he drove a tractor in the townland of Drumacabranagher, near Florencecourt. Later that year, on 23 September, McElwaine killed off-duty Royal Ulster Constabulary (RUC) Reserve Constable Ernest Johnston outside his home in Roslea. He was suspected of involvement in at least 10 other killings.