 He repossessed mining properties when owners defaulted on their loans, placing him in the mining industry.He made a fortune with copper mining, small smelters, electric power companies, newspapers, railroads (trolley lines around Butte and the San Pedro, Los Angeles and Salt Lake Railroad from Salt Lake City, Utah to San Pedro and Los Angeles, California), and other businesses, becoming known as one of three "Copper Kings" of Butte, Montana, along with Marcus Daly and F. Augustus Heinze.Between 1884 and 1888, Clark constructed a 34-room, Tiffany-decorated home on West Granite Street, incorporating the most modern inventions available, in Butte, Montana.