In the short story The Horses of Diomedes, Mertonshire is a fictional English county. Ashley Lodge, the home of General Grant, is located there as is the home of Beryl Larkin.
According to the text, Mertonshire a reasonable distance from London, and has all the attractions of fot country gentry: hunting, shooting and fishing, with several "very picturesque but slightly self-conscious" villages. It is well connected with railways and an arterial road connecting to London. Apparently "servant object to it less than they do to other, more rural" areas. Consequently Mertonshire had the reputation of being very expensive, requiring an income of "four figures, and what with income-tax and one thing and another, five figures is better."
It was "said that everyone has at least a second cousin in Mertonshire," However Poirot was an exception. Fortunately, when he needed to go there in the course of his investigations, he was able to rely on a friend Lady Carmichael.