Hillis was born in Pasadena, California, and received his bachelor's degree in zoology from the University of California, Berkeley in 1945. He then served in the United States Navy from 1945 to 1947. After his military service, he returned to Berkeley and received his master's degree in zoology in 1949 and his doctorate in zoology in 1952.
After completing his doctorate, Hillis joined the faculty of the University of Oregon, where he remained for the rest of his career. He was a professor of biology at Oregon from 1953 to 1993, and he served as the head of the Department of Biology from 1971 to 1977.
Hillis's research focused on the development of Drosophila, and he made several important discoveries in this field. He was one of the first researchers to use electron microscopy to study the development of Drosophila embryos, and he helped develop new techniques for staining and sectioning Drosophila embryos.
Hillis also wrote several books and articles on developmental biology, including the textbook "Developmental Biology of Insects" (1972). He was a member of the National Academy of Sciences and the American Academy of Arts and Sciences. He was also a fellow of the American Association for the Advancement of Science and the Society for Developmental Biology.
Hillis died of cancer in Eugene, Oregon, in 1999. He was 75 years old.