This name comes from Mac Mathuna, which means son of a bear. It was first used by the son of Murtagh Mór, an O'Brien king of Ireland, who died in A.D. 1l9. Traditionally, this sept was associated with County Clare: there's a 15th-century tomb of the McMahon family in Ennis Friary at Ennis, and at Loop Head the ruins of Carrigaholt Castle, a McMahon stronghold, can still be seen. The other sept of the MacMahons is connected with Co. Monaghan.

One noted McMahon was Maura Rua MacMahon, whose husband, Conor O'Brien, was killed by Cromwellian forces in 1651. They lived in Leamaneh Castle, near Kilfenora in Co. Clare. When enemy soldiers brought back her husband's body from the battlefield, she is said to have shouted at them from a window of the castle: "Take him away. We want no dead men here!"

In the l9th century, Edmonde Patrice MacMahon (1808-1893) became a President of France. The Chateau de Sully, near Orleans, is the home of the present Philippe MacMahon, the fourth Duke of Magenta and a direct descendant of the former President.