The restructured municipality of Southwest Middlesex was incorporated on January 1, 2001.

Southwest Middlesex had a population of 5,723 in the Canada 2016 Census.

The township includes the communities of Appin, Ekfrid, Glencoe, Lewis Corners, Macksville, Mayfair, Newbury Station, North Appin Station, North Ekfrid, North Glencoe Station, Riverside, Strathburn, Tait’s Corners, Wardsville and Woodgreen.

Wardsville was the site of the Battle of Longwoods during the War of 1812.

Canadian National Railway's Longwood Subdivision passes through Southwest Middlesex.