Collin County is a county in the U.S. state of Texas. As of the 2010 United States Census, the county's population was 782,341, making it the seventh-most populous county in Texas and the 63rd-largest county by population in the United States. Its county seat is McKinney. Collin County is part of the Dallas-Fort Worth-Arlington, Texas Metropolitan Statistical Area. A small portion of the city of Dallas is in the county.