On July 23, 1769, Gaspar de Portola led an expedition into what is now Orange County and camped a few miles east of the San Juan Capistrano area.

On July 24, the expedition headed inland to avoid the many streams and swamps in the area. They found a large plateau area and camped that night on its western edge by a canyon which the Franciscans named San Francisco Solano. This was on the eastern side of Trabuco Creek about three miles downstream from the present Trabuco Oaks.

While camped here on July 24-25, one of the soldiers lost his "trabuco" or musket, a most valuable possession to any soldier. To mark this loss, the stream was named Trabuco. The name has been associated with the mesa, the canyon and the entire area ever since.