On July 9, 1846, Montgomery and a party of 17 men landed and raised the first American flag[6] near the Mexican adobe custom house.

[5]: 22 Many historical events have happened at the plaza:[7] In the 1860s, the park was laid out with paved paths radiating out from the center, dividing the area into wedge-shaped lawns.

Trees were planted and had matured by the time the Square was adopted by the growing Chinese American community as Fa Yuhn Gok (meaning "the garden corner") in the 1880s.

[5]: 44–45  By 1905, many of the trees had been cleared and the Robert Louis Stevenson Memorial (erected in 1897) held a prominent spot in the park's central circle.

[5]: 65 A parking garage underneath Portsmouth Square was approved in early October 1959 by the San Francisco Board of Supervisors, over significant public opposition.

[8]: 9  The reinforced concrete garage was cast in place with parking floors and ramps designed to bear a load of 50 lb/sq ft (240 kg/m2) as required by building codes.

The second phase began in 1994, included installation of child play structures, Chinese chess tables, benches, and landscaping.

[14] The San Francisco Planning Department has initiated the Portsmouth Square Area Project to enhance the space and surrounding streets.

[16] A four-level underground parking garage is located beneath Portsmouth Square, accommodating an average of 50,000 vehicles per month.

[5]: 66, 72 [18] The Robert Louis Stevenson Memorial is also located inside the park to commemorate its history, designed by Bruce Porter and Willis Polk, and topped with a bronze galleon sculpted by George Piper.

[19] Tot Lot was commissioned by the city and county of San Francisco's Bureau of Architecture and two of the pieces, the ram and rabbit, were gifts of the Tamarack Foundation.

As the character "Dirty Harry" follows "Scorpio", it is possible to see the stone bridge[6] joining the park to the Chinese Culture Center of San Francisco, at 750 Kearny Street.