He played a prominent role in the Thirty Years' War, raising a regiment of 3,000 men, which served in both the Danish and Swedish forces.
[1] Donald Mackay took part in the subsequent operations together with his uncle Sir Robert Gordon, 1st Baronet, and with 300 men they proceeded as far as the town of Inverness in August, 1612 and again on 9 December 1613.
[1] In 1626, Sir Donald Mackay embarked with 3000 men at Cromarty under Count Mansfeld for the Thirty Years' War in the service of the king of Denmark alongside their colonel, Robert Monro.
[3] In March 1627 Sir Donald Mackay was created a Baronet of Nova Scotia,[4] and in 1628 was elevated in the peerage as Lord Reay.
[5] The following year in 1631, Lord Reay was empowered by Charles I of England to raise another force of men for service with Gustavus Adolphus, king of Sweden.
[6] He quarreled with David Ramsay at the English Court and, having challenged him to a duel, both were imprisoned in the Tower of London to preserve the peace.