[2] After joining the Royal Navy and serving as a midshipman, John Rainier was promoted lieutenant in May 1794 and was posted to HMS Suffolk in 1795.

[3][a] In April 1797 Rainier took command of HMS Centurion, a 50-gun ship of the line, initially remaining in the East Indies, but based in the Red Sea in 1799–1800.

This was after the British had received information that the French, as part of Napoleon's Egyptian campaign, had transferred ship frames to Suez to build warships for the Red Sea.

[10] The National Maritime Museum in London holds Rainier's Admiral's cocked hat[11] and his log books for the period 10 October 1796 – 14 February 1797 while he commanded HMS Swift.

After the election, the losing candidate, Major Charles Morgan, petitioned unsuccessfully against Rainier's return, alleging bribery, treating, corruption and improper use of a letter of support from a cabinet minister.

[14] Still a serving naval officer when elected, in January 1809 Rainier applied to the Admiralty for a month's leave when his ship was ordered for service abroad, he being 'very desirous to remain on shore in attendance on the House of Commons'.

[2] This made John Rainier a wealthy man by the standards of the time, especially when added to the prize money he had himself received during his naval service.