Little is known of Walker's art training. It is thought that he is to have studied painting in New York City. Upon leaving home in his early twenties, he went first to New Orleans and then to Mexico where he became interested in the Spanish-American culture. In 1846, war broke out between the United States and Mexico. He was imprisoned during the American siege in 1847 until escaping to the United States. He became an interpreter for General Winfield Scott in the U.S. Army and began sketching battle scenes. He became highly respected for later providing the Capitol in Washington with the large painting of the Battle of Chapultepec, which is housed at the Gilcrease Museum in Tulsa, Oklahoma, since 2006.

After the war, he established studios in New York and Washington, D.C., where he became well known as a painter of famous American battle scenes. He completed a number of government-commissioned works, one of which was placed in the Senate. In the late 1860s, or early 1870s, he settled into a studio in San Francisco. There he concentrated on genre scenes of the Mexican culture of early California. His highly detailed paintings realistically and accurately depict the costumes and gear of the rancheros and vaqueros on the cattle ranches.

Walker's works can be seen at the Los Angeles County Museum of Art, the Denver Art Museum, the U.S. Department of Defense Building, and in the Bancroft Library at the University of California, Berkeley.

James Walker died at the home of this twin brother in Watsonville, California on August 29, 1889.

Sources include:
Edan Hughes, Artists in California, 1786-1940
Charleston Renaissance Gallery, Charleston, South Carolina
Holly Watters, The Johnson Collection