Johngarthia lagostoma lives in burrows among vegetation, at altitudes of up to 400 m (1,300 ft), emerging at night to feed on plant matter and occasionally on animals.

The species was first described (as Gecarcinus lagostoma) by Henri Milne-Edwards in 1837 from material sent to him by the naturalists Jean René Constant Quoy and Joseph Paul Gaimard, collected by the French ship Astrolabe.

[2] 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).

[3] The geographic distribution of J. lagostoma across a small number of islands in the southern Atlantic Ocean is very unusual, and difficult to explain by planktonic dispersal.

[3] In 1915, H. A. Baylis reported that it feeds on "decaying vegetation and perhaps a certain amount of excreta from sea-birds";[6] most of its diet is thought to be made up of plant matter,[3] but can also include animals.

In the Wideawake Fairs, J. lagostoma feeds on chicks and eggs of the sooty tern, and they have been proposed as a limiting factor in the breeding of petrels on Ascension Island.

[2] In May, green sea turtles (Chelonia mydas) hatch, and J. lagostoma is one of several predators to target the emerging hatchlings, alongside the Ascension frigatebird (Fregata aquila) and feral cats.

[8] Bernard Stonehouse reported on an expedition by the British Ornithologists' Union to Ascension Island, stating:[13] We met land crabs, too, and were sadly disappointed.

They were shy, freezing with pincers erect when alarmed, usually to be found within a short distance of their burrows and ready to scuttle down at the drop of a hat.

Fringed mandibles [third maxillipeds] suggest a permanently turned-down mouth, giving them a disgruntled, unhappy expression; they were disagreeable rather than sinister and fell far short of their reputation.

The first land crab I met was sitting in a prickly pear bush, sadly munching one of the brilliant red fruits and dribbling juice.

Cherry-Garrard's book also includes a letter from Henry Robertson Bowers, dated Sunday, 31 July (likely 1910), which reads: "The land-crabs are little short of a nightmare.

"The closely related crabs from the islands of the Gulf of Guinea were formerly included in Gecarcinus lagostoma, but are now treated as a separate species, Johngarthia weileri.