The first mission, Mision San Antonio de Valero (The Alamo) was established in this area in 1718. Six years and 3 relocations later it moved to its final location on the east side of the San Antonio River at the site that we now know as The Alamo. Four other missions founded earlier, but in other parts of Texas, relocated to this area in 1731.
A Spanish military outpost, the Presidio San Antonio de Bejar, was also begun in 1718 to protect the mission.
The area was first created as a municipality in conjunction with the arrival of a new group of Spanish settlers from the Canary Islands on 9 March 1731. At that time it became known as San Fernando de Bexar and was under the authority of New Spain.
In 1811, San Antonio played an important role in the war between Mexico and Spain. Winning independence from Spain in 1821, the new Republic of Mexico invited settlers from the United States into Tejas. The Siege of Béxar in December of 1835 and the Battle of the Alamo in March of 1836 placed San Antonio at the forefront of the fight for the independence of Texas from Mexico.
Bexar County was formally established as one of the original 23 counties of the Republic of Texas on 17 March 1836. At the time, it included most of Texas to the north and west of the current county as well as parts of what are now New Mexico, Colorado, Oklahoma, Kansas, and Nebraska.
Texas and Bexar County retained their extended borders when Texas became the 28th state of the United States of America. In the Compromise of 1850, the borders of Texas were reduced to their current configuration in exchange for the federal government assuming Texas' pre-statehood debt. Bexar County's boundaries were reduced accordingly.
Between 1845 and 1876, roughly 128 new Texas counties were created from what was previously Bexar County's territory. A list of Texas counties with their dates of formation and parent counties is available at the Texas GenWeb site.