On July 7, 1846, the ships USS Savannah, Cyane and Levant captured the Alta (Upper) Californian capital city of Monterey without firing a shot. This procedure of occupation would set the stage for the Battle of Yerba Buena, which would follow a few days later.
On July 9, 1846, the USS Portsmouth, captained by Commander John B. Montgomery, sailed into San Francisco Bay, with the intent of capturing the town of Yerba Buena. Aboard the Portsmouth were 220 sailors and enlisted men, along with a contingent of 27 marines. The small Mexican force garrisoning the Presidio did not fire upon the USS Portsmouth, for fear that they would become obliterated. The Portsmouth landed a distance off the shore, and a group of sailors, soldiers and marines, as well as Montgomery and his staff, disembarked in rowboats for Yerba Buena.
Upon landing on the shore, Mexican soldiers held their fire, and the Californios grouped together to watch the American force. Montgomery and his force walked up to the flagpole in the town square, where the Mexican flag was flown. He quickly tore it down, and hoisted the Stars and Stripes in its place, proclaiming that the town of Yerba Buena, and all of the land surrounding it, belonged to the United States.
After Montgomery's speech, the marine band began to play Yankee Doodle, and the USS Portsmouth fired a 21 gun salute, to celebrate the capture of Yerba Buena. Following the capture of the town itself, Montgomery ordered a detachment of troops to seize the Presidio of San Francisco, and confiscate any weaponry they found, which the detachment did without conflict. Thus, the pueblo of Yerba Buena, and the land that would become eventually known as San Francisco, was captured by the United States of America.
The site was also home to the first public school built in California in 1847. Many other events occurred in the same area, since renamed Portsmouth Square in honor the USS Portsmouth:
On May 11, 1848, the discovery of gold was announced when Sam Brennan showed his gold to a crowd. On June 12, 1849, a crowd was gathered at the plaza, demanding election of delegates at the Monterey Constitutional Convention. An assembly was organized on July 16, 1849 to fight against a lawless body 'The Hounds.' On August 29, 1850, a memorial service was held after the death of President Zachary Taylor, also a war hero of the Mexican War. The First Admission Day celebration was held October 29, 1850 when California became the 31st state of the United States. On June 1, 1852, a crowd protested against the purchase of the Jenny Lind Theatre as the city hall. On September 18, 1859, Colonel E.D. Baker delivered an oration after U.S. Senator David C. Broderick was killed in a duel with California Chief Justice David S. Terry.
Director Don Siegal filmed a scene from the 1971 movie Dirty Harry in the Square. As the character Dirty Harry follows Scorpio it is possible to see the stone bridge joining the park to the Chinese Culture Center of San Francisco, at the Hilton Financial.
In 1987, the park underwent its second major renovation. The first phase involved installing new elevators and bathrooms on the top of the park. The second phase began in 1994, included installation of child play structures, Chinese Chess tables, benches, and landscaping. Phase three included the construction of a new community room and play areas. This $3.9 million renovation was completed and the park was opened to the public in 2001