Covenanter. He inherited his title from his father in 1635. In 1639, he went with James Graham, Earl (later Marquis) of Montrose, to bring the Covenant to the citizens of Aberdeen. He repelled James Gordon, Viscount Aboyne, near Stonehaven and, with Montrose, fought him again at the Bridge of Dee a few days later and was able to take Aberdeen. Keith showed some sympathy for King Charles I when he signed the 'Cumbernauld Bond', supporting royal authority against the extremes of Archibald Campbell, the Marquis of Argyll (1598 - 1661). However, Keith did not go as far as Montrose and join the Royalist army.
He would not meet Montrose in 1645, rather remaining inside his castle at Dunnottar. Montrose therefore burned his property, including the town of Stonehaven. The incident greatly effected the Earl and he eventually did support the King, at the Battle of Preston (1648) which turned into a rout, although Keith escaped. However, he was captured by Cromwell's forces at Alyth and held in the Tower of London for 9 years.
On the Restoration of Charles II, Keith was appointed to the Privy Council and as Keeper of the Privy Seal.