In 1898, after time spent serving in Gibraltar, St. John was posted on a Nile Expedition. This led to him seeing action at the Battle of Omdurman in Sudan. With the outbreak of the Second Boer War, St John was part of the 3rd Grenadier Guards sent to South Africa. On 23 November 1899, the 3rd Grenadier Guards were involved in the Battle of Belmont, and St John was amongst the troops pinned down at the base of Mount Belmont by Boer snipers on higher ground. When the decision was made to charge the position, St. John was at the forefront of his troop and engaged in hand-to-hand combat with the enemy. Reports vary widely, but it is stated that he killed between four and twelve combatants with his bayonet before his weapon became stuck in an enemy. On trying to retrieve his weapon he was shot in the temple and killed.