According to the United States Census Bureau, the city has a total area of 1.5 square miles (3.9 km2), all land.

As of the 2020 United States census, there were 747 people, 356 households, and 264 families residing in the city.

As of the census[2] of 2000, 839 people, 315 households, and 239 families resided in the city.

About 21.3% of all households were made up of individuals, and 12.1% had someone living alone who was 65 years of age or older.

The City of Mertzon is served by the Irion County Independent School District and home to the Irion County High School Hornets.