In The Case of the Ambitious Fishmonger, Captain Mackenzie was the factor (estate manager) of a large estate through which flowed the river Findhorn. He met Biggles at the office of Mr Sinclair of the Scottish Fisheries Board the morning after salmon poachers had struck at the Findhorn, leaving dead or dying fish all over the waters in his estate. When Biggles told Sinclair he needed a strong force of men to ambush the poachers at the Culbin Sands, Mackenzie willingly contributed his gamekeepers and ghillies.
In The Mystery of the Torn Parachute, Biggles had also met a Captain Mackenzie. It is likely that this is a different person since Sinclair introduced Biggles to Mackenzie and they did not show any sign of recognising each other.