Johngarthia lagostoma is found on Ascension Island, Trindade Island, Fernando de Noronha and the Rocas Atoll. On Ascension Island, J. lagostoma is restricted outside the breeding season to the slopes of Green Mountain, where there is sufficient moisture and vegetation, the rest of the island being too arid for the crab to survive. All the land above is suitable habitat for the crabs, as is much of the land above . They are occasionally found at lower altitudes, including the well-watered gardens of Georgetown, and the sooty tern breeding colony in the south-west of the island (known as the Wideawake Fairs). On Trindade Island, J. lagostoma is abundant wherever plants grow, included the highest parts of the island.