Jackling was born on August 14, 1869, near Appleton City, Missouri, the son of Daniel Jackling, a tugboat operator and trader, and Lydia Jane Dunn. His father died when Jackling was only a few months old; his mother, when he was not quite two. He was reared by his mother's sister, Abigail Dunn, who later married a Baptist minister, John T. Cowan. Danny Cowan, as he was called, grew up in a large family.
While attending local schools Jackling did farm work and other labor, and saved enough to put himself through two years of college. He enrolled at the state normal school in Warrensburg, Missouri, and resumed the name of Jackling, taking Cowan as his middle name. Working part-time at Warrensburg, Jackling managed to save enough during his freshman year to transfer to the Missouri School of Mines at Rolla. He was a brilliant student, and during his second and third years served as a special assistant to the professor of chemistry and metallurgy. A few months after Jackling received the B. S. and a degree in metallurgical engineering in 1892, the professor died. Jackling was appointed assistant professor and was placed in charge of the department of chemistry and metallurgy.
In 1893, Jackling worked for a few months at the Kansas City Consolidated Smelting and Refining Company's plant at Argentine, Kansas, then moved to Cripple Creek, Colorado. His work there as a chemist and metallurgist induced Joseph R. De Lamar, in 1896, to invite Jackling to take charge of the construction and operation of a gold recovery mill in Mercur, Utah. This involved a revolutionary gold metallurgy, mill construction, and application of high-potential electricity.
Jackling's work there during the next three years established him as a brilliant metallurgist and able administrator before he was thirty. In 1898, De Lamar asked Jackling and an associate, Robert C. Gemmell, to study the ore potential of properties at Bingham Canyon, Utah. Here lay a mountain of porphyry copper of such low grade that it would not pay to work it under contemporary technology. Jackling's report (September 1899) suggested a means of working the mine by opencut, mass-production procedures, and treating the low-grade porphyry ores by a similar mass-production process. Such a procedure had never been considered, and its later successful application made the report an international classic. Nevertheless, De Lamar was unwilling to pay the price upon which the owner of the property insisted, in addition to the heavy initial investment required before the copper could be reclaimed economically.
Jackling left Mercur for the state of Washington, where he built a plant employing the cyanide process to recover gold for Canadian capitalists. In 1901, Jackling moved to Colorado Springs, where he became associated with Charles M. MacNeill and Spencer and Richard Penrose, owners of a controlling interest in the United States Reducation and Refining Company, which operated two mills near Colorado Springs. As consulting engineer for the firm, Jackling was given the job of rebuilding and managing a zinc-pigment plant at Canon City, Colorado. But he could not forget the Bingham properties; and he succeeded in inducing MacNeill and the Penroses to buy the property, and with them organized the Utah Copper Company to develop it in 1903. He became vice-president and general manager of Utah Copper, and later president, and retained the last post until his retirement in 1942.
The Bingham operation was unprecedented - copper that yielded only twenty-five pounds per ton was successfully exploited. This success led to the development of other porphyry properties in the West. Jackling assisted in the development and operation of similar properties in Nevada under the Nevada Consolidated Copper Company, in Montana under the Butte and Superior Mining Company, in Arizona under the Ray Consolidated Copper Company, and in New Mexico under the Chino Copper Company. He served as either president or managing director of each of these companies. In order to make the properties accessible, railroads were constructed; Jackling was general manager of the Ray and Gila Valley Railroad Company in Arizona, the Bingham and Garfield Railway Company in Utah, and the Nevada Northern Railway Company. He was a director and chairman of the operating committee, and later president, of Kennecott Copper. In addition he was president of the Mesabi Iron Company and vice-president of the Alaska Gold Mines Company, president of the Utah Power and Light Company, and a director of Sinclair Consolidated Oil Company, Chase National Bank of New York, and a number of banks in Utah.
The revolutionary methods that Jackling introduced led to the development of porphyry copper deposits in Russia, Chile, and Africa. His enterprises in the West furnished approximately one-third of the copper produced in the United States during both world wars, as well as much gold, molybdenite, and silver. His adamant and forceful stand postponed the unionization of plants under his direction until the 1930's. Appointed honorary colonel of the Utah National Guard in 1909, Jackling used that title for the rest of his life. He died at Woodside, California, on March 13, 1956.
Jackling was a staunch Republican and a strong antiunion man.
Jackling married Virginia Jolliffe in April 1915; they had no children.
