Lancaster is a city in Dallas County, Texas, United States. Its population was 41,275 according to the 2020 census. Founded in 1852 as a fron…Lancaster is a city in Dallas County, Texas, United States. Its population was 41,275 according to the 2020 census. Founded in 1852 as a frontier post, Lancaster is one of Dallas County's earliest settlements. Today, it is a suburban community located in the Dallas–Fort Worth metroplex, about 15 mi south of downtown Dallas.