McKenzie is a non-canonical created for the film adaptation of The Plymouth Express' in Series 3 of ITV's Agatha Christie's Poirot. He murdered Flossie Carrington for her jewels and then used his accomplice Jane Mason, Flossie's maid, to dress up as her Mistress to create an alibi. Poirot came across his name while studying Miss Lemon's files and visited him incognito where he found a duplicate blue dress which Mason would have worn in order to impersonate Flossie Carrington. Based on this discovery, Japp was able to arrest him.
McKenzie was played by Kenneth Haigh. This character does not occur in the original story. There the jewel thief is "Red Narky".