Southlake is a city located predominantly in Tarrant County with minor areas extending into Denton County in the U.S. state of Texas. South…Southlake is a city located predominantly in Tarrant County with minor areas extending into Denton County in the U.S. state of Texas. Southlake is a suburb of Dallas/Fort Worth. As of the 2020 census, it had a population of 31,265.