Inspector NWMP/Lt. Reserve of Officers. Chalmers, a commissioned RMC graduate on the Reserve List, said to have been an "outstanding officer," was appointed 2 Troop Leader in D Sqn 2 CMR on formation of the Battalion and later promoted captain. During fighting around the Vet River during the first week of May 1900, Chalmers was detailed by Brigade Commander Hutton to seize the settlement of Pretorius, clearing the way to a crossing at Coetzee's Drift where the main action took place (5 May). Later in May he commanded two troops in Alderson's hard-riding Flying Column searching farms and skirmishing around Bothaville, north of Kroonstad towards Johannesburg where Chalmers and one of his men were particularly mentioned by General Smith-Doeien for "smart and aggressive scouting." He was engaged after that inall of the actions the Battalion saw up past Pretoria and towards Belfast.
In November in the days leading up to what turned into tthe Battle of Leliefontein, Chalmers was wounded while leading a party going to the aid of a group of comrades trapped under heavy fire above the Komati River. He had probably saved the life of Cpl Sherwood Herchmer, the son of the Battalion's original commanding Officer. Herchmer raced to Chalmers's aid, but he was too late and Chalmers bled to death.