The area was originally inhabited by native americans who were visited by a Spanish expedition in 1769. The area was under Spanish control until it became a part of Mexico. The governors created three ranchos: Rancho Trabuco, Rancho Mission Viejo, and Rancho Santa Margarita. In 1882 the area was purchased by James Flood and Jerome O’Neil, and run as a working ranch.
In 1940 the ranch was divided, and the O’Neill family retained the northern part. The Flood family parcel became Camp Pendleton in 1942.