[1] In 1846, President Polk offered Stevenson the command of a regiment of volunteers to be raised as part of the American occupation army during the Mexican–American War in California.

Stevenson with his Regiment of New York Volunteers sailed around Cape Horn for California on September 26, 1846, and arrived at San Francisco March 7, 1847.

[2] After his arrival in San Francisco, Stevenson joined General Stephen W. Kearny at Monterey and was made commander of the post.

He returned to San Francisco to enter the real estate business with Dr. William C. Parker, who had been assistant surgeon in the Regiment of New York Volunteers.

[5] He bought the southern half of Rancho Los Medanos in 1849 and laid out a site for a town they called "New York of the Pacific", which was changed to Pittsburg in 1911.