Americamysis almyra is native to shallow parts of the western Atlantic Ocean and Gulf of Mexico.

[1] Its habitat is typically seagrass meadows and shallow marshy areas at depths of less than 4 m (13 ft).

It is unclear whether it had invaded these areas, perhaps in the ballast water of ships, or whether it had been there previously but had been misidentified as Americamysis bigelowi.

[3] Americamysis almyra is an omnivore and examination of its stomach contents showed that its diet includes 31% vascular plant debris and 11% copepods and diatoms.

Opossum shrimps play an important part in the estuarine food chain and are consumed in large quantities by such fish as the striped bass, inland silverside, whiffs and flounders.