Cathcart joined the Royal Navy in August 1795 as a volunteer aboard Melpomene, a thirty-eight gun, fifth rate vessel originally captured from the French in 1794, being finally decommissioned in 1815.[4] Subsequently, he served on Pallas as a supernumerary (6 May – 6 August 1796), Able Seaman on Romulus (21 August 1796 – 13 January 1797), Midshipman (14 January – 12 November 1797), Midshipman and Master's Mate on Alcmene (13 November 1797 – 7 March 1799), and Midshipman on Majestic (8 March – 2 December 1799).[5] He was made lieutenant on 2 September 1801.