Janesville is a city in Black Hawk and Bremer Counties in the U.S. state of Iowa. The population was 829 at the 2000 census (approximately 85% in Bremer County). It is part of the Waterloo–Cedar Falls Metropolitan Statistical Area. According to the most recent estimates by the U.S. Census Bureau, and as cited by City-Data.Com,
the population of Janesville had increased to 876 in 2006.
According to the United States Census Bureau, the city has a total area of 1.5 square miles (3.9 km²), of which, 1.5 square miles (3.8 km²) of it is land and 0.04 square miles (0.1 km²) of it (2.68%) is water.
There were 349 households out of which 29.2% had children under the age of 18 living with them, 58.2% were married couples living together, 9.7% had a female householder with no husband present, and 29.5% were non-families. 25.8% of all households were made up of individuals and 12.3% had someone living alone who was 65 years of age or older. The average household size was 2.38 and the average family size was 2.83.
In the city the population was spread out with 22.7% under the age of 18, 7.7% from 18 to 24, 30.2% from 25 to 44, 23.5% from 45 to 64, and 15.9% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 100.7 males. For every 100 females age 18 and over, there were 98.5 males.
The median income for a household in the city was $40,060, and the median income for a family was $47,143. Males had a median income of $31,488 versus $21,481 for females. The per capita income for the city was $18,878. About 2.5% of families and 3.5% of the population were below the poverty line, including 3.2% of those under age 18 and 6.7% of those age 65 or over.
Janesville was founded in 1849 by John T. Barrick, a Quaker and abolitionist who had relocated to Iowa from Ohio. According to the book, "The Janesvillians, Volumes I and II" by Maxine Leonard, John T. Barrick built the first mill and frame house in the area. He platted the town of Janesville, which he named in honor of his wife, Jane McPherson Barrick.
It has been established that a tunnel once existed under the business district of Janesville. The tunnel ran between basements and below buildings on both sides of Janesville's Main Street, crossing below the street in the center of town and continuing westward to the Cedar River. One branch of the tunnel continued northward, connecting to the site of Fort John, a shelter built to protect settlers during the Ho-Chunk uprising in June, 1854.
The tunnel terminated in the basement of the home of Abel Crail, who later served in Union Army in the American Civil War, and was the first Commander of Janesville Post No. 172, Grand Army of the Republic. According to local legend, the Barricks and other townsfolk sympathetic to their cause aided in the escape of runaway slaves as part of the Underground Railroad. Slaves were moved through Janesville from Grinnell, Iowa and continued to Decorah, Iowa and into Southeastern Minnesota.
Janesville was a farming community of with a population of 311 in 1900, according to the Iowa Data Center
The town's population increased to just 445 by 1950. Due to its proximity to Waterloo-Cedar Falls, the population of Janesville increased to 840 by 1980, when the town was referred to as a "bedroom community." During the farm crisis and economic recession that hit Northeast Iowa in the 1980s, Janesville's population declined slightly. Since the mid-1990s, with the completion of the four lane bypass U.S. Highway 218 / Iowa Highway 27, known as the "Avenue of the Saints,"
Janesville's population is again increasing. New residential subdivisions continue to develop within the city of Janesville and the surrounding area.

Students at Janesville High School can choose to take advanced courses at nearby Waverly-Shell Rock High School in Waverly, the University of Northern Iowain Cedar Falls, and Hawkeye Community College in Waterloo. Wartburg College is located in nearby Waverly.
The community is served by the Janesville Public Library
1. Leonard, Maxine.The Janesvillians,Salem, MA: Hinningson, 1974 (reprinted 2006)