Hildreth's body of research includes work on the volcanic history of the Cascade Range, magmatism of the Long Valley Caldera, and mapping of mountain regions in the Andes.

[9][10] Between his sophomore and junior years, he joined an army reserve unit and trained for six months at Fort Ord,[4] earning the distinction "Outstanding Soldier of the Cycle" in 1959.

After graduating, he received a scholarship to travel the world, and he did for ten years, picking up a job as a naturalist for the National Park Service.

[9] Hildreth's interest in the Panamint Ranges led him to return to Death Valley and the Bishop Tuff while studying at Berkeley.

His analysis of the tuff was a major contribution to the field,[7] and since that time he has published on a wide array of geoscience topics, including volcanology, petrology, and geologic mapping, with a focus on continental formations such as calderas.

[9] In the 1970s, Hildreth saw a start to his career by studying the Bishop Tuff and Long Valley Caldera, and also by collaborating with Bob Christiansen on research in Yellowstone National Park.

[7] Prior to 1980, Hildreth's primary research partner was David A. Johnston, though he was killed by the 1980 eruption of Mount St.

Their collaboration began in 1980, when Hildreth took Fierstein—then a fresh college graduate—to the Valley of Ten Thousand Smokes in Katmai National Park and Preserve to conduct field research.

[9] Hildreth has served as an associate editor of Andean Geology since 1987, a role he previously held at the Journal of Geophysical Research from 1984 to 1986.

[2] The GSA awarded Hildreth and Fierstein the 2019 Florence Bascom Geologic Mapping Award (named for Florence Bascom) for their mapping efforts at Adams, Baker, Katmai, Laguna del Maule, and Long Valley as well as the Three Sisters, Simcoe Mountains, Pantelleria, Quizapu–Descabezado, and Mammoth Mountain.