In 1771 he went to St. Petersburg, where his father, Charles Cathcart, 9th Lord Cathcart, a general in the army, was ambassador. From 1773 to 1777 he studied law, but after succeeding to the lordship of parliament in 1776 he obtained a commission in the 7th Dragoons.
Proceeding to America in 1777, he had before the close of his first campaign twice won promotion on the field of battle. He transferred to the 17th Light Dragoons. In 1778 he further distinguished himself in outpost work, and at the Battle of Monmouth he commanded an irregular corps, the British Legion, with conspicuous success; for a time also he acted as quartermaster-general to the forces in America. He returned home in 1780, and in February 1781 was made captain and lieutenant-colonel in the Coldstream Guards.
He was elected a representative peer for Scotland in 1788, and in 1792 he became colonel of the 29th Foot. He served with distinction in the campaigns in the Low Countries, 1793-1795, in the course of which he was promoted major-general; and in 1801 he was made a lieutenant-general, having in the meanwhile received the appointments of Vice Admiral of Scotland (1795), privy councillor (1798), and colonel of the 2nd Life Guards (1797).
From 1803 to 1805 Lord Cathcart was commander-in-chief in Ireland, and in the latter year he was sent by Pitt in command of the British expedition to Hanover. After the recall of this expedition Cathcart commanded the forces in Scotland until 1807, when he was placed in charge of the expedition to Copenhagen, which surrendered to him on September 6. Four weeks later he was created Viscount Cathcart of Cathcart and Baron Greenock of Greenock in the peerage of the United Kingdom, resuming the Scottish command on his return from the front.
On January 1, 1812 he was promoted to the full rank of general, and a few months later he proceeded to Russia as ambassador and military commissioner. In the latter capacity he served with the headquarters of the allies throughout the War of Liberation (1812-1814); his success in the delicate and difficult task of maintaining harmony and devotion to the common cause amongst the generals of many nationalities was recognized after the war by his elevation to the earldom (July 1814). He then went to St. Petersburg, and continued to hold the post of ambassador until 1820, when he returned to England. He died at his estate near Glasgow on the 16th of June 1843.