The first known historical record of Mustang Island was made by Alonso Álvarez de Pineda, a Spanish explorer who charted the island in 1519. A fort was built on Mustang Island during the Mexican–American War, 1846–48, to guard the entrance to Aransas Bay. Records show that an Englishman named Robert A. Mercer and one H. L. Kinney both ran cattle on Mustang Island in the 1850s. During the Civil War, the coastal area was blockaded by the Union Navy, but the conflict ended with no major battles occurring in the area.