In 1947 he was hired by the University of California, Santa Barbara, where he taught in a growing department, soon joined by Wilbur R. Jacobs, Donald Marquand Dozer, C. Warren Hollister, Joachim Remak, Leonard Marsak, Frank J.
Frost, Robert O. Collins, Alfred Gollin, and Otis L. Graham, and many other scholars.
His research focused on the theme of the Spanish borderlands between Hispanic and Anglo-Saxon America and the earliest colonial history of the American Southwest.
Among his several influential books are Soldiers, Indians, and Silver: The Northward Advance of New Spain, 1550-1600 (1952) and Mexico's Miguel Caldera: The Taming of America's First Frontier, 1548-1594 (1977).
Powell died of a heart attack in Santa Barbara on September 17, 1987.