In 1794 he was appointed ensign in the 132nd Highlanders, from which short-lived corps he was promoted into the 89th Foot, and became captain-lieutenant in 1796.
He served with a flank battalion commanded by Colonel Stewart in the Irish rebellion of 1798, and afterwards with his regiment in 1799–1800 at the occupation of Messina after the blockade and capture of Malta under General Thomas Graham, Lord Lynedoch.
He served in the campaign in Egypt in 1801, including the battles before Alexandria, the defence of Rosetta, and the surrender of Cairo.
[3] After the fall of Alexandria the 89th embarked on board Lord Keith's fleet on a secret expedition, the destination of which was supposed to be Brazil; but on reaching Malta peace was found to have been declared, and the regiment returned to Ireland in 1802.
[11] John Le Mesurier died aged 62 on 21 May 1843 at Bradfield Place near Reading, Berkshire of apoplexy after a few days of illness.