[3] After joining the army, Montgomerie received a commission as a cornet in the Scots Greys.

[5] The regiment traveled to the American Colonies in 1757, and Montgomerie was put under the command of General Amherst.

[6] Montgomerie and his regiment, along with George Washington and Henry Bouquet, joined the expedition against Fort Duquesne in 1758.

Montgomerie's expedition, which included 1,200 men, was ultimately unsuccessful in its mission but succeeded in reaping mass devastation on lower Cherokee country.

He chose to give up Wigtown Burghs to sit in the seat for Ayrshire,[2] and served in the House of Commons from 1761 to 1768.

[14] The 10th Earl died in the early morning hours of 25 October 1769, and Montgomerie inherited the Earldom.

[6][4] Montgomerie was appointed Governor of Edinburgh Castle, in 1782,[8] and served as Lord Lieutenant of Ayrshire between 1794 and 1796.

[16] However, the majority of Archibald Montgomerie's wealth went to his daughter Mary,[8] whose son eventually became the 13th Earl of Eglinton.

It was offered back to the family by King William IV, but the 13th Earl declined.