After returning to San Francisco, Poston left for the East Coast in search of capital to fund a mining operation in the newly acquired territory. After several unsuccessful attempts, he was introduced to several Cincinnati, Ohio investors by Heintzelman. On March 24, 1856, US$2 million was secured to found the Sonora Exploring and Mining Company with Heintzelman as company president and Poston as managing supervisor. The company set up headquarters in Tubac, Arizona and began mining operations in the nearby Santa Rita Mountains and elsewhere. Poston served as alcalde of the settlement and became known as "Colonel" Poston in the town of roughly 800 people. Using the authority granted to him by the government of New Mexico Territory, he printed his own money and officiated over marriages, divorces, and the baptisms of children. This continued until Father Macheboeuf, the vicar of Bishop Jean-Baptiste Lamy, was sent from Santa Fe to investigate the goings-on in Tubac. The validity of the marriages Poston had performed was questioned by the vicar, and a US$700 donation made before Father Macheboeuf sanctified the unions. Charles Poston's brother, John, was murdered by Mexican outlaws at Cerro Colorado in southern Arizona, the town was established by Charles Poston.