Jefferson is a city in, and the county seat of Greene County, Iowa, United States, along the North Raccoon River. The population was 4,182 at the time of the 2020 census. It is the home of the Mahanay

Greene County is a county located in the U.S. state of Iowa. As of the 2020 census, the population was 8,771. The county seat is Jefferson. The county is named in honor of General Nathanael Greene.