Dexter is a city in Dallas County, Iowa, United States. The population was 640 at the time of the 2020 census. It is part of the Des Moines–Wes…Dexter is a city in Dallas County, Iowa, United States. The population was 640 at the time of the 2020 census. It is part of the Des Moines–West Des Moines Metropolitan Statistical Area. Dexter was founded in 1868.