John Francis O'Mahony was an Irish scholar and the founding member of the Fenian Brotherhood in the United States, sister organisation to the Irish Republican Brotherhood. Despite coming from a reasonably wealthy family and being well educated, the primary pursuit of O'Mahoney's life was that of Irish Independence from the United Kingdom, a calling that ultimately left him in poverty. O'Mahony fought in the Young Irelander Rebellion of 1848 as well as the US Civil War, and was involved organisationally in the Fenian Rising of 1867 in Ireland and the Fenian Raids on Canada.

The Fenian Brotherhood was an Irish republican organisation founded in the United States in 1858 by John O'Mahony and Michael Doheny. It was a precursor to Clan na Gael, a sister organisation to the Irish Republican Brotherhood (IRB). Members were commonly known as "Fenians". O'Mahony, who was a Gaelic scholar, named his organisation after the Fianna, the legendary band of Irish warriors led by Fionn mac Cumhaill.