In North America. Long-tailed Ducks breed in the Arctic and sub-Arctic and winter south along the Pacific coast to Oregon, the Atlantic coast to North Carolina, and the Great Lakes. The species is an uncommon migrant to Montana with fewer than 16 observations reported in the typical year (Robertson et al. 2002, MTNHP 2019).
For a comprehensive review of the conservation status, habitat use, and ecology of this and other Montana bird species, please see Marks et al. 2016, Birds of Montana.
During migration they are likely to be observed on reservoirs, lakes or large rivers, usually far from shore (Johnsgard 1986).