[1] He joined the 90th Foot in 1794, when, under Thomas Graham, (later Lord Lynedoch), he trained light company troops.
[2] On the death of Lieutenant-Colonel Ogilvie of the 44th Foot, Mackenzie transferred to that regiment, taking command.
[2] Injured after a fall from a horse in late 1803, he took leave, during which time he married Rachel Andrews, of Shorncliffe.
)[2] He saw some active service at Cadiz, but his health remained poor, and he spent most of the war convalescing in England.
In early 1814 he served under Thomas Graham in the Netherlands, commanding the 2nd Division in the Expeditionary Corps.