In 1745, Christie was gazetted Lieutenant of the 27th (Inniskilling) Regiment of Foot. Present with his uncle at the Battle of Culloden, young Christie was sent to the governor of Stirling Castle with the despatches announcing the victory. As Captain of the 44th Regiment of Foot, in 1756 General Abercrombie appointed him assistant-deputy-Quartermaster-General to the Forces in North America, maintaining that position under the new commander-in-chief, Earl Loudoun. He fought with Wolfe at the Siege of Quebec, when he was promoted Brevet Major. In 1762, he was promoted to Lieutenant-Colonel, and in 1769 he was made deputy-Quartermaster-General to the Forces in North America. In 1773, he was appointed Lt.-Colonel of the King's Royal Rifle Corps and served with them in the West Indies. In 1778, he was made Colonel Commandant of the 2nd Battalion of that regiment. In 1780, they were serving in Antigua and in 1781 he was appointed Major-General and Commander-in-Chief of the Leeward Islands. In 1793, he was gazetted Lt.-General, and in 1796 he was made Colonel Commandant of the King's Royal Rifle Corps. In 1798, he received his final promotion to General and Commander-in-Chief of the British Forces in the Canadas, at Montreal.