He volunteered into the 2nd Battalion, Canadian Mounted Rifles for service in South Africa during the Second Boer War in January 1900, as captain, and was promoted major in May 1900.

[3] After returning to Canada, he was in April 1902 appointed to command the Western Regiment of the fourth Canadian contingent which left for service in South Africa the following month.

Sir Archie had been awarded the Order- Knight Commander, The Most Honourable Order of the Bath (military division) [hence the post-nominals, KCB] for his service in the First World War.

Pierre Berton described MacDonnell in the book Vimy: A Presentation General Officer sword, which he was awarded in May 1919 by the officers of the 1st Canadian Division, 'The Older Patch' is in the collection of the Royal Military College of Canada virtual museum [8] From 1919 to 1925, he was appointed commandant of the RMC.

[10] The Sir Archibald Macdonell Athletic Centre or SAM Centre, which opened in 1974, at the RMC in Kingston, Ontario is named after Lieutenant-General, Sir A.C. Macdonell, KCB, CMG, DSO, C de G. His cocked bi-corn hat, a form of headdress worn by Colonels, General Officers and Staff Officers, with red and white feathers for Generals, is on display at the Kingston Military Community Sports Centre.

The personal coat of arms of Lt Gen Sir Archibald Cameron Macdonell was carved on the Currie Building at the Royal Military College of Canada.