Jefferson is a city in Greene County, Iowa, United States, along the North Raccoon River. The population was 4,626 at the 2000 census. It is the county seat of Greene County. It is the home of the Mahanay Memorial Bell Tower, 168 feet (51 m) tall, located on the town square, and visible for miles. The tower is named for Floyd Mahanay, a philanthropist and former Jefferson resident.