[5] Illinois Route 10 passes through the city as Chestnut Street, leading east 19 miles (31 km) to Lincoln and west 8 miles (13 km) to Easton.

Illinois Route 29 touches the western edge of the city, leading north 29 miles (47 km) to Pekin and south 32 miles (51 km) to Springfield, the state capital.

According to the U.S. Census Bureau, Mason City has a total area of 1.01 square miles (2.62 km2), all land.

There were 1,127 housing units at an average density of 1,145.1 per square mile (442.1/km2).

31.5% of all households were made up of individuals, and 18.3% had someone living alone who was 65 years of age or older.