Mesa County is a county located in the U.S. state of Colorado. As of the 2020 census, the population was 155,703. The county seat and most populous municipality is Grand Junction. The county was named for the many large mesas in the area, including the Grand Mesa, which is the largest flat-topped mountain in the world. Mesa County comprises the Grand Junction, CO Metropolitan Statistical Area. In 2020 it ranked as the 271st most populous metropolitan area in the United States.