According to the United States Census Bureau, the city has a total area of 1.0 square miles (2.5 km²), all of it land.
Mason City is situated halfway between Peoria and Springfield on Route 29, a north-south highway, and halfway between Lincoln and Havana on Route 10, an east-west road.
There were 1,041 households out of which 30.4% had children under the age of 18 living with them, 51.3% were married couples living together, 11.2% had a female householder with no husband present, and 34.5% were non-families. 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. The average household size was 2.38 and the average family size was 2.95.
In the city the population was spread out with 25.2% under the age of 18, 7.0% from 18 to 24, 25.3% from 25 to 44, 20.7% from 45 to 64, and 21.9% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 87.1 males. For every 100 females age 18 and over, there were 81.7 males.
The median income for a household in the city was $35,615, and the median income for a family was $47,991. Males had a median income of $35,058 versus $21,875 for females. The per capita income for the city was $18,411. About 6.8% of families and 9.3% of the population were below the poverty line, including 13.6% of those under age 18 and 8.0% of those age 65 or over.