The city is bordered by Escondido to the east, Encinitas to the southwest, Carlsbad to the west, and Vista to the northwest.

Lake San Marcos is an enclave, or county island, within the southwestern part of the city and its sphere of influence but is an unincorporated community.

According to historical legends, the San Luis Rey Mission flocks were robbed by a small band of Native Americans in the late 18th century.

While pursuing the Native Americans in 1797 the Spaniards came upon a fertile valley, which was named Los Vallecitos de San Marcos (Little Valleys of Saint Mark) to honor the day of discovery: April 25, St. Mark’s Day.

In the late 1850s, Soto sold part of his land to Cave Couts, and his family was soon raising livestock.

Although Cave Couts owned the land, Major Gustavus French Merriam from Topeka, Kansas, made the first permanent settlement.

Merriam homesteaded 160 acres (0.65 km2) in the north Twin Oaks Valley and began wine and honey production.

In 1883, a few miles south of the settlement, John H. Barham (for whom the present-day Barham Drive is named) founded the first town in the area, originally situated on the southeast corner of what are now Rancho Santa Fe Road and San Marcos Boulevard.

By 1884, the town of Barham had a post office, blacksmith, feed store and weekly newspaper.

[citation needed] The original town of San Marcos was about a mile north of Barham, at the intersection of what is now Grand Avenue and Rancho Santa Fe Road.

By 1896, San Marcos was a community with its own stores, post office, blacksmith, and railroad depot.

To prevent San Marcos from suffering a similar fate, in 1903 the people of the town picked up their homes and moved a couple miles east along the railroad tracks to what now are Mission Road and Pico Avenue.

San Marcos experienced a period of growth from 1956 onward, when the first water from the Colorado River arrived.

[9] According to the United States Census Bureau, the city has a total area of 24.4 square miles (63 km2).

Due to the moderating influence of the nearby Pacific, temperatures rarely fall below freezing in winter and above 100 degrees F. in summer.

San Marcos has a diversity of races and ethnic groups common in suburban communities (not ranked, but for example): African Americans, Irish, Italian, Portuguese, German, Polish, Russian, Armenian, Iranian, Indian, Vietnamese, Chinese, Japanese, Filipino, Mexican, Salvadorian, Argentine, Cuban, Puerto Rican, Hawaiian, Jewish, and Samoan.

[21] San Marcos uses a council-manager system of government with a separately elected mayor.

SMUSD serves more than 21,000 students in San Marcos, as well as sections of Vista, Escondido, Carlsbad and Encinitas, and some unincorporated areas of the county, such as Buena, Twin Oaks Valley, Deer Springs, Lake San Marcos, Harmony Grove, and Elfin Forrest.

It provides adult education services in the City of San Marcos and its surrounding communities.

It works with Palomar College and other local adult schools to connect students with post-secondary education and training.

A giant letter P (for "Palomar") on the hillside above campus is a San Marcos landmark, visible miles away.

State Route 78 provides freeway access to San Marcos, with six interchanges serving the city.