Leptocereus grantianus is found on the island of Culebra in the subtropical dry forest life zone. Mean annual rainfall ranges from a minimum of 6OO to a maximum of 1,OOO millimeters in this life zone. The vegetation tends to form a complete ground cover and is almost entirely deciduous on most soils. Leaves are often small and succulent or coriaceous, and species with thorns and spines are common. Tree heights do not usually exceed 15 meters and crowns are typically broad, spreading, and flattened.
Fire is common and successional vegetation primarily includes grasses.
Leptocereus grantianus is located in dry thickets which grow on a crumbling rock substrate on a steep bank just above the shoreline. Historically, deforestation and selective cutting for agriculture, grazing, production of charcoal, and the cutting of wood for construction materials have affected dry forest vegetation. Currently, the island of Culebra is subject to intense pressure for residential and tourist development. Land adjacent to the population is currently proposed for the development of a housing project. Its ornamental potential may result in take becoming a problem in the future.