The long-tailed duck was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae.

[3] Linnaeus cited the English naturalist George Edwards's description and illustration of the "Long-tailed duck from Hudson's-Bay" that had been published in 1750 in the third volume of his A Natural History of Uncommon Birds.

[10] An undescribed congener is known from the Middle Miocene Sajóvölgyi Formation (Late Badenian, 13–12 Mya) of Mátraszőlős, Hungary.

Small numbers are found on the Missouri river Adults have white underparts, though the rest of the plumage goes through a complex moulting process.

They are migratory and winter along the eastern and western coasts of North America, on the Great Lakes, coastal northern Europe and Asia, with stragglers to the Black Sea.

According to the Audubon Society Field Guide to North American Birds they can dive to 80 fathoms (146 metres or 480 feet).

There has been a significant decline in the number of birds wintering in the Baltic Sea, partly due to their susceptibility to being trapped in gillnets.