Shortly after his father's death in 1841, he began working as a clerk in a small general store in New York City at the age of 15. At age 21, he moved to Buffalo, New York, at the invitation of his cousin, Elihu J. Townsend, and became the cashier of the Merchants' Bank of Erie County, and later a one third owner. In December 1848, he took an exploratory trip to California, through the Isthmus of Panama, where he joined the California Gold Rush, following two of his brothers, James and Edgar Mills. By November 1849, he had made $40,000 and decided to make California his permanent home. Therefore, in 1850, he returned to Buffalo where he sold his interest in the Bank and returned to Sacramento, where he founded his own bank, the "Gold Bank of D. O. Mills & Co." This was helped significantly by a cousin from the English branch of the Mills family, Charles Mills, 1st Baron Hillingdon, who ran the Glyn, Mills & Co. bank in London. He never invested in gold mining or silver mining directly, as he considered mining to be too speculative. He rather started ancillary businesses that supported the mining industry, such as banks and railroads. He was a part owner of the Virginia and Truckee Railroad, which was the only link from the Comstock Lode to the Central Pacific Railroad. The major share holder in the railroad was William Sharon, whom William Ralston had sent to Virginia City as representative of the Bank of California. In 1864, with other investors, he founded the Bank of California, which grew large in the 1860s and 1870s, but collapsed due to financial irregularities involving its chief cashier, William Chapman Ralston. Mills used his personal fortune to revive the bank, along with Sharon, and attract new investment, and within three years, the bank was again strong.
Later life
In 1880, two years after resigning from his second term as the president of the Bank of California, Mills returned to New York, where he participated in the development of a number of buildings in Manhattan, including 160 Bleecker Street, or "Mills House No. 1". He also invested in the Niagara Falls Power Company, one of the first large power companies organized in the United States. His devotion to philanthropy involved sitting on the boards of a number of charitable and cultural institutions.
Personal life
On September 5, 1854, he married Jane Templeton Cunningham, the daughter of Elizabeth Griffiths and Scottish born James C. Cunningham, who was a pioneer and shipowner. Together, they had a son and a daughter:
Ogden Mills, who married Ruth T. Livingston, granddaughter of Maturin Livingston
Elisabeth Mills, who married Whitelaw Reid, the U.S. Ambassador to Great Britain.
