John William Nixon, MBE (1880 – 11 May 1949) was a unionist politician in Northern Ireland who was alleged to be responsible for several atrocities, including the McMahon killings. Born in Graddum, a townland located between the village of Kilnaleck and the hamlet of Crosskeys in County Cavan, Nixon became a district inspector in the Royal Irish Constabulary (RIC), and transferred to its successor in the then newly created region of Northern Ireland, the Royal Ulster Constabulary (RUC). By 1922, he was responsible for controlling access to the Roman Catholic Ardoyne and "Bone" areas of Belfast, and worked closely with the Ulster Special Constabulary.