Fannin County is a county in the far northeast of the U.S. state of Texas, on the border with Oklahoma. As of the 2020 census, its population was 35,662, making it the 87th-most populous county in Texas. The entirety of Fannin County is a part of the Bonham micropolitan statistical area and the Dallas-Fort Worth combined statistical area. The county seat is Bonh…Fannin County is a county in the far northeast of the U.S. state of Texas, on the border with Oklahoma. As of the 2020 census, its population was 35,662, making it the 87th-most populous county in Texas. The entirety of Fannin County is a part of the Bonham micropolitan statistical area and the Dallas-Fort Worth combined statistical area. The county seat is Bonham.