Mr Mellon is a young member of Poirot's and Major Porter's club and the Detective's friend. He lives in the vicinity of Warmsley Heath and is acquainted with Jeremy Cloade.
He enjoys "creating alarm and despondency in such places as it is not forbidden by the Defence of the Realm Act". When Jeremy Cloade, until then unnoticed, leaves the club after hearing Porter's story about Robert Underhay, Mellon gleefully remarks that the solicitor might sue Porter for defamation of character. He also (correctly) guesses that the story will soon reach the ears of the rest of the Cloade clan who shall promptly try to use it to their advantage. Mr Mellon then tenderly escorts Poirot out of the club, noting how terrible the atmosphere in these establishments is, largely due to a high concentration of "club bores" such as Major Porter.
Two years later, when Poirot is employed by Rowley Cloade to search of a man who knew Robert Underhay, he briefly consideres getting in touch with Mellon and ask him for help. Then, he finally remembers Major Porter's name, and dismisses the idea.