Frank Leonard Sizer was born in Illinois, in September 1856. He graduated from the University of Michigan in 1878, and then traveled to Leadville, Colorado, to begin a career as a civil engineer. Sizer arrived in Helena, Montana Territory, on March 3, 1882, to serve as the chief clerk for the surveyor general's office. After leaving the surveyor general's office, Sizer assumed a series of positions including engineer with the Sun River Canal Company constructing an irrigation system (1884); general manager of the British-owned Empire Mining Company at Eldorado Bar (1886-1888); partner in Sizer and Keerl (James S.), a Helena mining engineering firm (1890-1897); consulting engineer to Helena mining brokers, Canoll and Martin; and chief of the irrigation division of the Department of Agriculture for Washington (1893), a position he received largely because of the support of T.C. Power. In private practice, Sizer maintained an office in Helena's Pittsburgh Block and an office in Butte, where he enjoyed a long business relationship with William A. Clark. In 1900, Sizer testified as an expert witness in the famous Colusa Parrott Mining and Smelting Company v. Anaconda Copper Mining Company case in which Clark filed suit against Anaconda for exploitation of adjoining veins. During 1901, Sizer spent nearly a year at the Rosario Mine in Chihuahua, Mexico, in the employ of Clark. In May 1903, as a result of his examinations there, Sizer recommended the purchase of Rosario Mines, Limited, to Charles W. Clark, William Clark's son. Sizer also had interests in many other mining ventures through the years including the April Fool properties (in Nevada), Moonlight Mine (at Silver Star), Eclipse-Argo Mining Company (at Canyon Ferry), Gould Mining Company, Missouri and Montana Gold Mining Company, Whitlatch Mining Company, and Spring Hill Mining Company. The Whitlatch-Union Mine at Unionville, developed by James W. Whitlatch, was the first gold quartz mine discovered in Montana. In 1904, Sizer examined the mine and, impressed with its potential, he traveled East with mining broker, H.N. Canoll, to find funding to purchase the property and to finance the sinking of a new shaft. Canoll and his partner, A.W. Martin, obtained a lease to the property from the National Mining and Exploring Company and then transferred their options to the newly formed Whitlatch Mining Company. This firm was largely funded by a group of Chicago businessmen including Jacob Bauer, Charles Minshall, and Joseph, Edward, and Louis Mohr. Martin remained an officer in the Whitlatch Mining Company, but Canoll sold his interests in 1905. Sizer served as the company's general manager and devised plans for a 30-stamp mill, electrical plant, and a cyanide processing plant. Late in 1906, the Whitlatch Mining Company attempted to buy some adjoining property, the Spring Hill Mine, which belonged to the Pittsburgh and Montana Copper Company. When the Whitlatch firm could not raise funds for the purchase, the property was obtained by a new company, the Spring Hill Mining Company, organized by James T. Stanford of Great Falls and George L. Ramsey. After lengthy negotiations, the Whitlatch Mining Company ceased operation and in 1907 agreed to lease its property to the Spring Hill Mining Company. Sizer became the general manager of the Spring Hill operation which also mined the Whitlatch and the Twilight Group. The properties were worked for less than two years, producing a limited tonnage of low-grade gold ore, and the venture was never financially successful. In 1909, Frank Sizer, his wife Linda, and their children, Randolph, Margaret, Miriam, Patricia, and Milton, moved to California. Sizer died on July 11, 1942, at the age of 85.