Williamson County is a county in the U.S. state of Texas. As of the 2020 census, its population was 609,017. Its county seat is Georgetown. The county is named for Robert McAlpin Williamson, a community leader and a veteran of the Battle of San Jacinto. Williamson County is part of the Austin-Round Rock-San Marcos metropolitan statistical area. It was included with Austin in the Best Cities to Live in for 2009 by the Milken Institute.