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 1,852 at the 2010 census. Of this, 1,354 were in Clark County, and 498 were in Marathon County. The city is bordered by the Town of Colby, the Town of Hull, and the City of Abbotsford.