The main justification for a separate species designation is the size difference between the two minks, but other distinctions have been made, such as its redder fur.

The sea mink was first described in 1903, after its extinction; information regarding its external appearance and habits stem from speculation and from accounts made by fur traders and Native Americans.

It was probably found on the New England coast and the Maritime Provinces, though its range may have stretched further south during the last glacial period.

Conversely, its range may have been restricted solely to the New England coast, specifically the Gulf of Maine, or just to the nearby islands.

[10] But a 2001 study by Graham concluded that this size difference was insufficient evidence to classify the sea mink as its own species and that it should be considered a subspecies.

[14] A 2021 study into New World weasels found that the sea mink, along with four other extant species, should be classified into a new genus, Neogale.

[15] Fur traders who hunted it gave the sea mink various names, including water marten, red otter, and fisher cat.

[14][17] The sea mink was a marine mammal that lived around the rocky coasts of New England and the southernmost Maritime Provinces until it was hunted to extinction in the late 19th or early 20th century.

[16][17] The bones of a specimen unearthed in Middleboro, Massachusetts, were dated to be around 4,300±300 years old, 19 kilometers (12 mi) from salt water.

[7] During the last glacial period, ending 12,000 years ago, the sea mink's range may have extended south of the Gulf of Maine.

Its relatives, as well as descriptions by fur traders and Native Americans, give a general idea of this animal's appearance and its ecological roles.

[14] The type specimen was collected by Prentiss and Frederick True, a biologist, in 1897 in Brooklin, Maine, the remains of which consist of a maxilla, parts of the nasal bone, and the palate.

[6][14] These minks were large and heavily built, with a low sagittal crest and short, wide postorbital processes (projections on the frontal bone behind the eye sockets).

Mead, concluding that the mink was restricted to nearshore islands, suggested that the large size was due to insular gigantism.

[5] As marine mammal species often play a large part in their ecosystems, the sea mink could have been an important intertidal predator.

[7] Remains of toad sculpins and ocean pout were the most common around their dens, and garden banded snails were also reported to have been part of their diet.

If a sea mink escaped into a small hole on the rocky ledges, it was dug out by hunters using shovels and crowbars.

[9][14][18] One study looking at the remains in shell middens in Penobscot Bay reported that sea mink craniums were intact, more so than that of other animals found, implying that they were specifically placed there.