The Masilonyana Local Municipality (FS181) was established in term of the Municipal Structure Act and the Municipal Demarcation Act. The municipality forms part of Lejweleputswa District Municipality (FS184). The municipality head office, Theunissen is situated 102 kilometers from Bloemfontein and approximately 55 kilometers from Welkom. The municipality comprises of former five Transitional Local Councils, that is, Theunissen, Brandfort, Winburg, Verkeerdevlei and Soutpan.

The municipality covers an area of 679608.6ha of which 77.7 % has remained as natural habitat. There is one formal land-based protected areas in the municipality, namely the Sandveld Nature Reserve (1740.9ha). There are no RAMSAR sites in the municipality. There is one biome in the municipality, namely Grassland. Nine vegetation types, are found in the municipal area. These are Bloemfontein Karroid Shrubland, Central Free State Grassland, Eastern Free State Clay Grassland, Highveld Alluvial Vegetation, Highveld Salt Pans, Vaalbos Rocky Shrubland, Vaal-Vet Sandy Grassland, Western Free State Clay Grassland and Winburg Grassy Shrubland.

There is one endangered ecosystem, namely Vaal-Vet Sandy Grassland covering 68029.2ha (10 %) of the municipality. There is also one threatened ecosystem, namely Eastern Free State Clay Grassland covering 1.18 % of the municipality. Seven rivers run through the municipality, namely the Groot Vet, Klein Vet, Korannaspruit, Laaispruit, Modder, Taaibosspruit and Vet Rivers. Wetlands cover 19470.7ha (2.9%) of the municipal area.