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In the short story The Double Clue, Mr Johnston is a South African millionaire who was present at a tea party hosted by Marcus Hardman where a valuable necklace of his was stolen.

According the Hardman, Johnston had just rented the Abbotburys' house in Park Lane. He was very wealthy and Hardman thought it was unlikely he could have been the thief. Poirot and Hastings did not manage to see Mr Johnston when they called at the Park Lane house. His secretary told them Mr Johnston had only recently come from South Africa. He was not so much interested in collecting jewels as he was in gold.


Mr Johnston is played by Nicholas Selby in the film adaptation of The Double Clue in Series 3 of ITV's Agatha Christie's Poirot. There he becomes "Mr Johnstone" and he has the first name "Martin". Poirot does not meet Mr Johnstone but Hastings and Miss Lemon do interview him during their own investigation.