Johnston was based at the branch of the USGS in Menlo Park, California, but his work on volcanoes took him all over the Pacific Northwest region. When the first earthquakes shook Mount St. Helens on March 16, 1980, Johnston was nearby at the University of Washington, where he had pursued his doctorate. Intrigued by the possible advent of an eruption, Johnston contacted Stephen Malone, a professor of geology at the university. Malone had been his mentor when Johnston had worked at the San Juan complex in Colorado, and Johnston admired his work. Malone stated that he "put him to work" almost instantly, allowing Johnston to escort interested reporters to a place near the volcano. Johnston was the first geologist on the volcano, and soon became a leader within the USGS team, taking charge of monitoring of volcanic gas emissions.