Colby is a city in Clark and Marathon counties in the U.S. state of Wisconsin. It is part of the Wausau, Wisconsin Metropolitan Statistical Area. The population was 1,952 at the 2020 census. Of this, 1,275 were in Clark County, and 677 were in Marathon County. The city is bordered by the Town of Colby, the Town of Hull, and the City of Abbotsford. Colby cheese was first created in Colby.