San Fernando is a city located in the Mexican state of Tamaulipas; it serves as the seat of the surrounding municipality of the same nameā¦San Fernando is a city located in the Mexican state of Tamaulipas; it serves as the seat of the surrounding municipality of the same name. It is about 85 miles away from Brownsville, Texas, United States. The municipality has a population of 57,220, while the city itself has a population of 29,665.