In 1851, Poston traveled to California as part of the Gold Rush and took a clerk position at the San Francisco Customs House. While at this job, he became involved with a group of French bankers interested in the lands of the recently negotiated Gadsden Purchase. In late 1853, with the bankers' backing, Poston joined with mining engineer Herman Ehrenberg to organize an expedition into the territory Mexico was expected to sell to the United States. Taking a ship from San Francisco, the two became shipwrecked near the Mexican port of Guaymas. They were then detained temporarily by Mexican authorities as suspected filibusters before heading north to the Gadsden territory. The expedition visited San Xavier del Bac and Ajo, collecting mineral samples along the way, before traveling down the Gila River. At Fort Yuma, a U.S. Army post near the confluence of the Gila and Colorado River, Poston first met the fort's commander Major Samuel P. Heintzelman.
Poston commissioned Tiffany & Co. to create a US$1500 inkwell from Arizona silver and presented the inkwell to Lincoln upon signing of the Arizona Organic Act. On 12 March 1863 Poston was appointed superintendent of Indian affairs. This appointment was followed by his election as Delegate to the U.S. House of Representatives on 18 July 1864. During his term of service, Poston submitted bills aimed at settling private land claims and to establish Indian reservations along the Colorado river.
Poston also wrote several books during this time, publishing The Parsees in 1872, The Sun Worshippers of Asia in 1877, and his poem Apache Land in 1878. His work, Building a State in Apache Land was published in installments by Overland Monthly between July and October 1894.
He was made register of the United States land office at Florence, Arizona from July 1877 till June 1879. During his time in Florence he became interested in building a Parsi fire temple on a nearby hill, paying for construction of a road to the summit and petitioning the Shah of Persia for funds to build the temple. This unusual interest led to Poston being criticized as a crank and eccentric.
Following his time in Florence he moved to Tucson and supported himself with a variety of positions including lecturer, mining and railroad promoter, and writer. Charles D. Poston filed a claim on land which he called "Hole-in-the-Rock" in 1892. The land was set aside as the Papago Saguaro National Monument in 1914 and in 1930, became Papago Park.
Poston declined into obscurity until 1897 when Whitelaw Reid published an account detailing Poston's situation. As a result, the Arizona Territorial legislature awarded Poston a pension of $25/month in 1899 and increased this to $35/month in 1901. Poston died in poverty on June 24, 1902 and was buried in Phoenix, Arizona. Poston’s remains were removed from Phoenix and moved to Florence, Arizona, on the 100th anniversary of his birth. His remains were buried on Primrose Hill, renamed "Poston Butte" where he had never completed his “Temple to the Sun”. He was entombed in an official ceremony led by Governor George W. P. Hunt.
