In October 1849 Rich accepted a call to assist Amasa Lyman in supervising Mormons in California. Between 1851 and 1857 Rich and Lyman established a relatively prosperous economic colony at San Bernardino, which served as a way-station for immigrants traveling to Utah via the Spanish Trail. Recalled in 1857, Rich moved back to Centerville. He represented Davis County in the territorial legislature and served as aide to General Daniel Wells of the Nauvoo Legion during the Utah War. Between 1860 and 1862 Rich joined Lyman in England to oversee the Mormon Church's European Mission.
After a one-year respite in Centerville, Rich accepted Brigham Young's call to colonize the Bear Lake region against the threat of non-Mormon settlement. In September 1863 Rich led his party from Franklin, Idaho, into Bear Lake Valley, settling at present-day Paris, Idaho. In 1864 Rich moved his six wives and thirty children to Paris and began a twenty-year struggle to maintain the colony in the face of severe winters, poor harvests, delicate Indian relations, and isolation. In 1864 Brigham Young honored Rich by naming Rich County, Utah, and the town of St. Charles, Idaho, after him.