Keith began his mining career at the age of fourteen in the gold mines of Nova Scotia. At the age of twenty, he left Nova Scotia, traveling to California by ship via Panama. He moved on to Virginia City, Nevada, where he worked as a miner and became proficient as a pump operator in the water-plagued mines of the Comstock Lode. His skill with mine pumps brought him to Park City in 1883 to help install the great Cornish pump; and he then continued as foreman in the Ontario mine. During the eight years he worked as foreman, the Ontario mine paid seven million dollars in dividends.
In Park City Keith became business partners and good friends with Thomas Kearns, and in 1892 they organized the Silver King Mining Company with Keith serving as president and Kearns as vice-president. By 1907 the Silver King mine had paid more than ten million dollars in dividends. Keith and Kearns participated in other joint business ventures, including purchase of the Salt Lake Tribune in 1901. The two men owned the newspaper until Keith's death.
David Keith was active in many other business enterprises including the Pioneer Roller Mills and Keith-O'Brien Company. He helped finance the construction of the David Keith Building, the Ness Building, and the Little Hotel. He was president of the First National Bank of Park City, vice-president of the National Copper Bank of Salt Lake City, and director of the Las Vegas and Tonopah Railroad as well as the Los Angeles and Salt Lake Railroad.
David Keith was married twice. Little is known about his first wife, who bore him four children--Charles, Etta, Lillie, and Margaret. Rumor was that she left him for another man, and though she tried to return to her position as wife and mother, Keith could not forgive her. In any event, the four children remained with their father, who married Mary Ferguson in 1894. A native of Salt Lake City, Mary had taught school in Park City and then had become manager of the local telephone company. One child, David, was born to Mary.
Between 1898 and 1900 the Keiths build a magnificent mansion, designed by Frederick A. Hale, at 529 South Temple in Salt Lake City. They lived in the mansion until 1916 when they moved to the Hotel Utah. David Keith died there of pneumonia on 16 April 1918, and Mary died thirteen months later of a heart ailment on 17 May 1919.