Pasco was born on 20 December 1774 (some accounts say 29 December) and entered the Royal Navy on 4 June 1784 as captain's servant, on . In 1786 he moved to , under the command of the then Prince William Henry (later
William IV), spending about 12 months on duty in the
West Indies and along the coast of North America. He served on various ships after 1787 and by 1790 he had risen to the rank of
midshipman under Captain John Manley on . He continued serving on various ships as midshipman and
master's mate until 1795, mainly in the
English Channel and West Indies. On 15 July 1795 he was promoted to the rank of
lieutenant and assigned to . In 1796 he joined under Lancelot Skynner (who, three years later, captained
HMS Lutine on her last voyage). During his service aboard
Beaulieu he assisted in the capture of
St Lucia and on 27 December 1799 he became
first lieutenant aboard . In 1801, while still on
Immortalite, he volunteered to command a boat to cut out the French
corvette Chevrette from
Camaret Bay on the north coast of
Brittany; however, bad weather frustrated the plan. On 7 April 1803 he was assigned to Nelson's
flagship . While serving on
Victory he assisted in the capture of the French 32-gun
frigate , was involved in the blockade of
Toulon and pursued the French squadrons in the West Indies. ==Trafalgar==