Mobile City is a city in Rockwall County, Texas, United States on the outskirts of Rockwall. The population was 188 at the 2010 census. It currently has the highest population density for any city in Texas, and is the only Texas city that is on the highest population densities of American cities list. It currently ranks 74th on the list, and has the lowest population for any city on that list. It is tied with Poplar Hills, Kentucky as the smallest city by total area on the list.