He was the eldest son of George Keith, 5th Earl Marischal and his wife, Margaret (d 1598), daughter of Alexander Home, 5th Lord Home. He and his father were summoned on 1608 to answer for William"s conduct to Francis Sinclair, son of the earl of Caithness. Keith acted as his father"s deputy in the Scottish parliament in June 1621 and succeeded to his father"s earldom on the latter"s death on 2 April 1623.
He ratified his father"s building of Marischal College by a charter of 1 October 1623 and attended James VI and I"s funeral on 5 May 1625 at Westminster Abbey.
He left for the continent in 1601, travelling for his education to Paris, Orléans, Tours and Saumur. He also joined the Scottish council of war on 12 July 1626 and was put in command of three ships to transport troops to aid Christian IV of Denmark, Charles I"s uncle. This caused tension with Scotland"s lord high admiral, Alexander, earl of Linlithgow, but these were in the end resolved by Marischal"s confirmation as commander of the king"s navy in Scotland on 9 July 1631.
He received Charles I on his entry into Edinburgh in 1633 after his coronation and then in 1634 fitted out another fleet, which he sent to the assistance of Władysław IV Vasa.
He was made a baronet of Nova Scotia at the end of May 1625 and, on the new Scottish privy council"s restructuring in March 1626, he was made a member of lieutenant Isobel, married to Sir Edward Turnor (Member of Parliament for Orford).
