Throughout the distribution range, the Least Weasel occurs in different habitats – from Arctic tundra to sand deserts and alpine mountains (up to 3,860 m asl). Habitat selection is usually determined by the local distribution of rodents. It is often found in open forests, farmlands and cultivated fields, grassy fields and meadows, riparian woodlands, hedgerows, alpine meadows and forests, scrub, steppes, semi-deserts and deserts, prairies, and coastal dunes (Sheffield and King 1994). It is well adapted to tundra and snowy environments and ranges above the tree line in Arctic and alpine areas. Snow presents a little obstacle to the distribution, hunting, or movements of M. nivalis. Least Weasel is a specialist predator of small mammals (especially rodents), although it also occasionally feeds on birds’ eggs, fishes, lizards, amphibians, invertebrates and carrion. Small rodents (voles, mice, lemmings, etc.) are the main diet for M. nivalis, but it can successfully attack and kill animals much larger than itself. Least Weasels are sufficiently small to follow and kill small rodents in confined spaces, such as narrow tunnels and burrows. In northern regions, the Least Weasel spends part of the year hunting under the snow, searching for vole and Lemming nests in the subnivean environments.

The Least Weasel is a solitary animal – no pair bond between adults. Males and females live separately for most of the year, except during the breeding season. Home range is around 1–30 ha, and varies with sex and region (King and Powell 2007). The size of the home range depends upon habitat, population density, season, and prey density. Females spend more time hunting in rodent tunnels and can therefore live on disproportionately small home ranges. Den sites may include burrows taken over from prey, holes in tree trunks or under tree roots through forest habitats or in piles of logs, ditches, and isolated patches of scrub in open habitats. Breeding may continue throughout the year but is concentrated in spring and late summer. In the Arctic zone, when small rodents are abundant, M. nivalis can breed during winter under the snow. When prey is abundant, adults can produce a second litter in late summer or autumn, and early–born young females can produce their first litter at this time. No delayed implantation (in contrast to M. erminea). Gestation is 34–36 days. The litter size averages 4–5, range 1–11. The family splits up after 3–4 months. Most wild individuals do not survive 1–2 years, but captives have lived up to 10 years.

The Least Weasel has a very large circumboreal distribution, covering most of Europe. It occurs throughout the Palearctic, from the British Isles (except Ireland) eastward to the Urals and including some Arctic islands. The Least Weasel has been introduced to many Mediterranean islands and to the Azores.

Outside Europe, this species is found in northern Asia, North Africa and North America, from Siberia to Afghanistan, Mongolia, eastern China, and Korea. It is also found on some Pacific islands: Sakhalin and Kuril islands (Shumshu, Paramushir, Kunashir), the Japanese islands (Hokkaido and northern Honshu), and Taiwan. It is distributed in North America from Alaska south to the Appalachian Mountains. It has been introduced to Sao Tome off West Africa and New Zealand; populations in North Africa (Morocco, Algeria and Tunisia) also were introduced by humans (Lebarbenchon et al. 2010, McDevitt et al. 2012, Rodrigues et al. 2017).

This species is found in many protected areas. It is listed on Appendix III of the Bern Convention (Pulliainen 1999) and is protected under national and sub-national legislation in a number of range states. Monitoring is required to quantify the population trend in Europe.