His brother William Alexander was a Member of Parliament,[2] whilst his sister Harriet was the wife of the High Church clergyman John Edward Nassau Molesworth.
In 1809, the Coldstream Guards embarked for the Iberian Peninsula, and was present in all the great battles there against the Napoleonic forces, beginning with Talavera and ending with Toulouse.
Early in June 1815, he embarked with another officer at Ramsgate in an open boat, in order to join his regiment who were quartered near Brussels, and arrived the next morning at Ostend.
In advancing to charge the French, leading a portion of his regiment, he received a shot in his knee which killed his horse and in falling he lost his sword.
Notwithstanding the pain of his wound, and his leg being almost disabled, Colonel Mackinnon continued to defend that perilous post until the advance of the whole British line, and the subsequent rout of the French Army, put an end to the struggle of the day.
[6][5] In 1826, he purchased the majority in the Coldstream Guards which gave him the rank of full colonel in the British Army, and the command of the regiment to which he had been attached all his military life.