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. In March 1627 Sir Donald Mackay was created a Baronet of Nova Scotia, and in 1628 was elevated in the peerage as Lord Reay. In 1630 Donald Mackay, 1st Lord Reay accompanied his regiment to Germany, and was present at the capture of Stettin and Colberg. 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. 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. During 1632 Gustavus Adolphus, king of Sweden was killed at the Battle of Lützen and Lord Reay was not repaid large sums of money due to him by the king.