He succeeded to the titles of 7th Lord Kennedy and 5th Earl of Cassillis on 14 December 1576.
He was one of the central figures in The Historie Of The Kennedyis , an anonymous eyewitness account of the feud between the Cassilis and Bargany Kennedy families republished in 1830 by Robert Pitcairn. This feud climaxed in a pitched battle in December 1601 in which the Earl's men fatally wounded the Laird of Bargany who was returning home from Ayr. Other dark deeds committed by the Earl according to the Historie include the summary hanging of a young man named Dalrymple, to whom he was related, for being a closer relation of Bargany's. In most these deeds he was heavily influenced by his Tutor Laird Thomas Kennedy of Culzean, which led to the murder of Culzean by Bargany's younger brother Thomas Kennedy of Drummurchie at Greenan Castle on 12 May 1602.