Carson's notoriety grew as his name became associated with several key events in the United States' westward expansion. He was still serving as Fremont's guide when Fremont joined California's short-lived Bear-Flag rebellion just before the outbreak of the Mexican-American War in 1846, and it was Carson who led the forces of U.S. General Stephen Kearney from New Mexico into California when a Californio band led by Andrés Pico mounted a challenge to American occupation of Los Angeles later that year.

At the end of the war, Carson returned to New Mexico and took up ranching. By 1853, he and his partner were able to drive a large flock of sheep to California, where gold rush prices paid them a handsome profit. This same year Carson was appointed federal Indian agent for Northern New Mexico, a post he held until the Civil War imposed new duties on him in 1861.