John Mitchel was born at Camnish near Dungiven in then County Londonderry, in the province of Ulster. His father, Rev. John Mitchel, was a Non-subscribing Presbyterian minister of Unitarian sympathies, and his mother was Mary (née Haslett) from Maghera. From 1823 until his death in 1840, John Sr. was minister in Newry, County Down. In Newry, Mitchel attended a school kept by a Dr Henderson whose encouragement and support laid the foundation for classical scholarship that at age 15 gained him entry to Trinity College, Dublin. After taking his degree at age 19 he worked briefly as a bank clerk in Derry, before entering legal practice in the office of a Newry solicitor, a friend of his father.