The O'Mahony name comes from Mathghamban, son of Cian Mac Mael Muda, a 10th-century prince, and his wife Sadbh, daughter of the Irish High King Brian Boru. The O'Mahonys have always been particularly prominent in Munster, especially in Co. Cork, where their castles were strung out along the coastline as far west as Mizen Head. Many O'Mahonys still live there.

Among the prominent names in the clan was John O'Mahony (1816-1877), from Kilbeheny, near Mitchelstown in Co. Cork, who was cofounder of the Fenian movement. Also in the 19th century, the renowned Fr Francis O'Mahony, known as "Father Prout" was the author of such works as The Bells of Shandon. Eoin O'Mahony (1904-1970) was a barrister by profession, a great raconteur and broadcaster, who frequently discussed clan history on the radio; he was a familiar bearded figure travelling the roads of Ireland.

Today, the chief representative of the O'Mahony clan in Europe is Vicomte Yves O'Mahony of Orleans, France.