He joined the Scientific Corps of the United States Exploring Expedition of 1838 as botanist, and was in the 1841 overland party headed by George F. Emmons that traveled south from the Oregon Territory along the Siskiyou Trail to upper California, before rejoining the expedition's ships in San Francisco.
[1][4][5] Rich "found escape from having to report on the expedition's botanical collections"[1] in the outbreak of the Mexican–American War.
In November, 1846, he was appointed paymaster, with the rank of major, to the troop of United States volunteers sent to subdue California.
[4] The volunteer troops were disbanded when the war ended in 1848, but Rich continued to serve as paymaster until his honorable discharge on October 31, 1849.
[1] Rich Passage, a tidal strait in Puget Sound, was named in his honor by Charles Wilkes, leader of the Exploring Expedition.