Owing to a rough crossing of the Atlantic that necessitated a stop-over in the West Indies, he did not arrive to take up the post until April 1769.
[3] Mackay was promoted to lieutenant-general on 6 September 1777 and became Governor of Landguard Fort on 14 April 1778 after the death of Sir John Clavering.
[7][8] On 13 May 1780 he was appointed Commander of the forces in North Britain, replacing Sir James Adolphus Oughton, and instigated reforms to reduce expenditure and remove sinecures.
In this capacity he proposed a postal service for Sutherland, assisted the kelp and fishing industries and constructed the pier at Eriboll.
[1] He resigned his seat in March 1773, by the procedural device of accepting appointment to the sinecure of Steward of the Chiltern Hundreds.