Pangio longimanus is presently only known from the type locality - the confluence of 2 streams in central Laos, the Nam Leuk and Nam Gnong. However, the species should be expected elsewhere in the Mekong drainage.
In the dry season, Nam Leuk is a small stream a few metres wide and usually shallow (rarely more than 1m deep). The Nam Gnong is an even smaller stream.
The Nam Leuk and Nam Gnong meet where the Nam Leuk enters the Mekong floodplain. At the confluence, the Nam Leuk widens into a somewhat circular pool, about 50m in diameter (depth unknown).
Since the 3 fish that led to the new species description were collected, the Nam Leuk dam was constructed upstream of the type locality. How this may have affected Pangio longimanus is unknown.
Small, shallow streams where the substrate is sand with occasional rocks, and mud with beds of leaf litter.
P. longimanus was collected from the type locality in leaf litter, at about 1m depth.