Located between the Esperance Plains, Avon Wheatbelt and Coolgardie bioregions, it has a low, gently undulating topography, a semi-arid mediterranean climate, and extensive Eucalyptus mallee vegetation.

Recognised as a region under the Interim Biogeographic Regionalisation for Australia (IBRA), it was first defined by John Stanley Beard in 1980.

It has an area of about 79000 square kilometres (31000 mi2), making it about a quarter of the South West Botanic Province, 3% of the state, and 1% of Australia.

Further east, the area is virtually uninhabited, except for Lake King, and a few small towns along the Coolgardie-Esperance Highway south of Norseman, such as Grass Patch and Salmon Gums.

[3][4] The region has a low, gently undulating topography, with somewhat occluded drainage, resulting in a series of playa lakes.

As with other regions in semi-arid areas of the South West, it exhibits high endemism, especially with respect to Eucalyptus and Acacia species.

[8] Approximately 56% of the Mallee region falls within what the Department of Agriculture and Food calls the "Intensive Land-use Zone", the area of Western Australia that has been largely cleared and developed for intensive agriculture such as cropping and livestock production.

The remaining 44% of the region falls within the "Extensive Land-use Zone", where the native vegetation has not been cleared but may have been degraded by the grazing of introduced animals and/or changes to the fire regime.

It cannot be distinguished in Ludwig Diels' 1906 regionalisation of the South West, and is treated as part of the Wheatbelt in Edward de Courcy Clarke's 1926 map.

It first appeared in Beard's 1980 phytogeographic regionalisation of Western Australia, but with an eastern limit of Point Culver.