This species occurs in damp habitats with dense understorey vegetation. It has been found in forest, woodland, heathland, tussock grassland, and sedgeland. It prefers lower elevation areas, with a southerly aspect and moderate slope (Wilson and Bachmann 2008).

The Swamp Antechinus is endemic to Australia. The subspecies A. m. maritimus (Finlayson 1958) is found as fragmented populations in near-coastal areas of south-eastern South Australia and Victoria west of and including Sunday Island and Wilsons Promontory. Isolated populations in southern Gippsland Plain have been most affected by habitat modification and/or alienation. In South Australia, recorded since 1970 from coastal lakes and swamps south of Robe. In Victoria, recorded since 1970 from three main areas, the south-western Wannon region, the Otways, and the southern Gippsland Plain (Menkhorst 1995, Bachmann and van Weenen 2001).

A. m. minimus (Geoffroy Saint-Hilaire 1803) is present on Tasmania and the Bass Strait Islands (Wilson and Bachmann 2008).

The Swamp Antechinus is present in a few protected areas; most of the mainland population is found in protected areas (and, indeed, there have been a few efforts to protect and maintain suitable habitat in grazed areas in South Australia). Much of the species' range in the western half of Tasmania is in a World Heritage Site. There is a need for a systematic survey of the current range of A. m. minimus to obtain accurate distribution baseline and determine current threats, as has recently been done for A. m. maritimus in South Australia (Bachmann and van Weenen 2001), as the subspecies is probably undergoing declines and the amount of suitable habitat available is likely to be small.