[11] The following year Mackenzie went to South Africa on the outbreak of the Second Boer War, and from 1900 he served as Director of Intelligence on the staff of the Commander-in-Chief, Lord Roberts.
In a despatch dated 31 March 1900, Lord Roberts described how Mackenzie "afforded … material assistance by the accurate and valuable reports he submitted".
[13] For his service in the war, Mackenzie was appointed a Companion of the Order of the Bath (CB) in the April 1901 South Africa Honours list (the award was dated to 29 November 1900[14]), and he received the actual decoration after his return home, from King Edward VII at Buckingham Palace on 24 October 1902.
[15] In September 1902 he received the substantive rank of lieutenant-colonel,[16] and was appointed Assistant Quartermaster General for the 5th Division, within the 2nd Army Corps, based in Dover as part of the staff of the South-Eastern military district.
[7] His departure from that post was caused in part by a disagreement between Mackenzie and Sam Hughes, the Canadian Minister of Militia and Defence, as to (among other things) the merits of the Ross rifle.
[7] He took over the 3rd Division following the death in action of Major-General Hubert Hamilton; however he only lasted for two weeks in this post before he was relieved of his command following the inconclusive result at the Battle of La Bassée in October 1914.
[28][29] Mackenzie himself was wounded by an enemy sniper on 27 April 1918 while he was visiting the line of the 183rd Brigade south of St. Floris, being shot through the cheek and parotid gland.