 Its county seat is the city of Dallas, which is also Texas' third-largest city 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. Polk.Dallas County is included in the Dallas-Arlington-Fort Worth metropolitan statistical area (colloquially referred to as the Dallas-Fort Worth metroplex).