He became an excellent horseman in Russia then from 1773 to 1777 he studied law at the University of Glasgow and was afterwards called to the Scottish Bar.

In 1778 he further distinguished himself in outpost work, and at the Battle of Monmouth he commanded the British Legion, with conspicuous success; for a time also he acted as quartermaster-general to the forces in America.

[3] In September 1784 Cathcart organised the first formal cricket match in Scotland, in the grounds of Schawpark and commissioned a family portrait by David Allan to commemorate the event.

[3] From 1803 to 1805 Lord Cathcart was Commander-in-Chief in Ireland, and in the latter year he was sent by Pitt to supersede Sir George Don in command of the 14,000 strong British Hanover Expedition.

[3] He occupied Hanover on 14 December and joined with Werdereffsky's Russian column of Tolstoi's corps.

[3] On 1 January 1812 he was promoted to the full rank of general, and a few months later he proceeded to Russia as ambassador and military commissioner.