Jefferson is a city in Greene County, Iowa, United States, along the North Raccoon River. The population was 4,345 at the 2010 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 businessman, philanthropist, and former resident. Jefferson is bisected east to west by the old Lincoln Highway (formerly U.S. 30). A new U.S. 30 is located about one mile to the north.