At the age of fourteen he entered the army as an ensign in the Coldstream Guards, and shortly after accompanied the regiment to Bremen. In 1807, the battalion to which he belonged sailed for Copenhagen, and after the capture of that city it returned to England.
Peninsula
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. Having attained the rank firstly of lieutenant, then Captain, Mackinnon was appointed aide-de-camp to General Sir Edward Stopford, and distinguished himself throughout the campaign by his cool daring. On one occasion, when the army was passing a defile where British troops were debouching from it under destructive fire, the troops found Captain Mackinnon coolly shaving himself in a spot where the danger was greatest. Encouraged by this, the soldiers rushed forward and drove the French before them.
Waterloo
In 1814 Mackinnon was promoted to the rank of lieutenant colonel. 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. He was present at the engagements of the 16th and 17th, and at the Battle of Waterloo on the 18th, where he had three horses shot from under him. 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. He fell close beside a French officer who was even more severely wounded, and in taking the latter's sword gently told him he hoped they might sup together that night. On recovering his legs he again mounted, urging on his men, advancing at their head. In the latter part of the day Colonel Mackinnon was ordered to occupy the farm of Hougoumont, where he was placed with about 250 of the Coldstream Guards and the first regiment of the Grenadier Guards. Aware of the great importance of this position, flanking the British Army, the Duke of Wellington sent orders that it be defended to the last. On this point, Napoleon directed his efforts, ordering battalion after battalion to the assault, with terrific carnage. 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 route of the French Army put an end to the struggle of the day. When the action was over, Mackinnon collapsed from loss of blood and fatigue and was sent on a litter to Brussels to recover. 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.
Military Author
King William IV had expressed a desire that every officer in command of a regiment should furnish the Horse Guards with some account of it, and Colonel Mackinnon wrote a work that was to become well-known: The Origin and Services of the Coldstream Guards, which was published in 1833.
