He was the eldest son of Sir Hugh O'Donnell, the ruler of Tyrconnell for much of the reign of Elizabeth I.
He was a leading contender in the O'Donnell succession dispute of the 1580s which took place while his father was still alive.
Amongst the other leading contenders were his half-brother Hugh Roe O'Donnell and his great-uncle Hugh Dubh O'Donnell. Donnell received the backing of the Crown, who felt he had a strong claim to the title although his father had not nominated him as his successor. Shortly after the Armada shipwreck of 1588 he was appointed as Sheriff of Donegal by the Lord Deputy William FitzWilliam.
Fitzwilliam also had Sir Eoin O'Gallagher, an influential supporter of Hugh Roe arrested and imprisoned. Donnell grew stronger in the late 1580s, and took control over much of western Tyrconnell. She hired large numbers of Redshank mercenaries from her native Scotland to confront her son's rival.
Donnell was defeated and killed at the Battle of Doire Leathan on 3 September 1590. Two years later Hugh Roe successfully had himself made Chief of the O'Donnells.
He was also powerful enough to interfere in a succession dispute in neighboring Fermanagh, where he established Hugh Maguire as chief ahead of his rival Connor Roe Maguire.
