For statistical purposes, the United States Census Bureau has defined the community as a census-designated place (CDP).

There is considerable evidence of human inhabitation of the area long before the Spanish establishment of the Mission San Diego de Alcala in A.D. 1769.

Native American tribes lived and flourished near the San Luis Rey River in the Late Prehistoric period.

[3] Much of this area was originally part of Rancho Vallecitos de San Marcos.

On April 22, 1840, Governor Juan Bautista Alvarado granted the 8,877-acre Rancho Los Vallecitos de San Marcos to Don Jose Maria Alvarado who married Lugarda Osuna, daughter of the owner of San Dieguito Rancho, Don Juan Maria Osuna.

It is a resort-based community surrounding the Lake San Marcos reservoir.

According to the United States Census Bureau, the CDP has a total area of 1.8 square miles (4.7 km2).

The lake after which the area is named is a large artificial body of water originally created for purposes of irrigation and watering cattle; today, however, it is used primarily for recreational and commercial purposes.

The community attracts a large number of retired and elderly residents though over time younger families with children are moving in.

[8] In the California State Legislature, Lake San Marcos is in the 40th Senate District, represented by Republican Brian Jones, and in the 46th Assembly District, represented by Democrat Jesse Gabriel.