Mason is an incorporated town in Effingham County, Illinois, United States. The population was 345 at the 2010 census, down from 396 at the 2000 census. It was named after Roswell Mason, an qualified of the Illinois Central Railroad. Mason is ration of the Effingham, IL Micropolitan Statistical Area.

Mason is located in southern Effingham County at 38°57′9″N 88°37′35″W / 38.95250°N 88.62639°W / 38.95250; -88.62639 (38.952398, -88.626490). Illinois Route 37 passes through the town, leading northeast 6 miles (10 km) to Watson and southwest 3 miles (5 km) to Edgewood. Interstate 57 crosses the northwest corner of Mason, but the closest access is from Edgewood to the southwest or from Exit 151 4 miles (6 km) to the northeast. Via I-57 it is 14 miles (23 km) north to Effingham, the county seat, and 55 miles (89 km) southwest to Mount Vernon.

According to the 2010 census, Mason has a total Place of 1.295 square miles (3.35 km), of which 1.29 square miles (3.34 km²) (or 99.61%) is land and 0.005 square miles (0.01 km) (or 0.39%) is water.

As of the census of 2000, there were 396 people, 147 households, and 103 families residing in the town. The population density was 348.3 inhabitants per square mile (134.5/km²). There were 160 housing units at an average density of 140.7 per square mile (54.3/km). The racial makeup of the town was 98.23% White, 0.25% Native American, 0.76% from other races, and 0.76% from two or more races. Hispanic or Latino of any race were 2.27% of the population.