Foshag received his bachelor's degree from the University of California, Berkeley in 1919 and then joined the U.S. National Museum, now called the National Museum of Natural History. After joining the National Museum he earned his Ph.D. in 1923 from U.C. Berkeley with the thesis The origin of the colemanite deposit of the western United States. As head curator of the Museum's Department of Geology, Foshag greatly increased the Museum's mineral collections and its collections of gemstones. During his tenure and in large part due to his efforts, the Museum received both the Washington Roebling and Frederick Canfield mineral collections, which ranked among the world's best private collections of minerals and gems collections in the world — the Roebling collection had about 16,000 specimens. The Roebling and Canfield gifts included large endowments of money. From 1926 to 1941 Foshag's work in Mexico was largely funded by the Roebling Fund. Much of Foshag's fame stems from his study of the Parícutin volcano, which for the first time gave scientists an opportunity to study a volcano's entire life cycle. Foshag was in Mexico in 1943 when the Parícutin volcano first appeared. He remained in Mexico for more than two years documenting the growth of Parícutin. In 1946 Foshag and Edward P. Henderson traveled to Japan to examine gemstones confiscated by the U.S. Army; the two experts spent more than four months there sorting and appraising gemstones, worth some $25 million in 1946 U.S. dollars. Foshag and Henderson met Mikimoto Kokichi at Japan. Based upon his discovery, in Guatemala's Motagua Valley, of the only in situ jadeitite locality south of the U.S. border, Foshag in 1949 was commissioned by the Guatemalan government to survey jade objects from pre-Columbian Central America.