The long-tailed duck has a circumpolar distribution and breeds throughout Alaska, northern Canada, Greenland, Iceland, Svalbard, Fennoscandia, the Kola Peninsula and Siberia. The species nests both in mountainous regions and in lowlands, often far from the coast. It is thought to be migratory, but its movements are very poorly documented. The long-tailed duck breeds mainly in the western parts of Spitsbergen and on Bjørnøya within Svalbard. Solitary pairs breed on small islands and along the coast of Spitsbergen, close to freshwater ponds. Observations from the mid 1990s suggest that at least some individuals from the Svalbard population winter in ice-free waters off the west coast of Svalbard, mainly along Nordenskiöldkysten and Prins Karls Forland. Long-tailed ducks can be seen along the ice-edge early in spring. Important wintering areas are located along the coast of northern Norway and in the Baltic.

The long-tailed duck breeds on the tundra, usually near freshwater ponds and lakes. Several pairs may nest close together though the species is not regarded as being a colonial breeder. The nests are often placed in colonies of arctic terns, which affords them protection against predators such as the artic fox, glaucous gull and arctic skua. Incubating female long-tailed ducks remain on the nest until the last possible moment when an intruder approaches, similar to common eider females. Studies performed in Ny-Ålesund showed that incubating females left the nest three or four times during each 24 hour period. The female usually flew out to sea, but was back after only three or four minutes. The eggs are covered with down when left, but are still vulnerable to predators. The males are very territorial and occupy the same area every year. The territory usually includes a freshwater pool where intense courtship behaviour occurs. Outside the breeding season long-tailed ducks stay mainly in coastal waters. They show considerable variation in habitat use and dive to various depths. Their diet consists mainly of invertebrates, but they also feed on some plant matter. In Svalbard long-tail ducks eat mussels, crustaceans and insect larvae.

The long-tailed duck starts breeding as soon as the breeding area is free of ice in late June. The nest is lined with dark-coloured down that has white speckles; the nest itself is a shallow depression directly on the tundra. In Svalbard there is little vegetation to provide cover, but the brown plumage of the incubating females makes them very difficult to detect. The five to nine pale grey-green eggs are laid in the latter half of June. The female incubates the eggs for four weeks. The males leave the females early in the incubation period and go out to the coast where they moult either singly or in small flocks. The young follow the female to a nearest freshwater pond shortly after hatching. They stay here for some time, before moving out to sea. The young are fledged after five to six weeks and become independent of the adults. The females moult somewhat later than the males.

The size of the Svalbard breeding population is not known, and there is no monitoring of population trends. The segment of the breeding population that winters off the west coast of Spitsbergen is vulnerable to oil spills and disturbance if these occur because they are energetically challenged in winter, and many birds gather together in a restricted area at the same time.