Dallas County is a county located in the U.S. state of Texas. As of the 2010 census, the population was 2,368,139. It is the second-most populous county in Texas and the ninth-most populous in the United States. Its county seat is Dallas, which is also the third-largest city in Texas and the ninth-largest city in the United States. The county was founded in 1846 and was possibly named for George Mifflin Dallas, the 11th Vice President of the United States under U.S. President James K.