Mary Jane Osborn was born in Colorado Springs, Colorado, and raised in west Los Angeles and Beverly Hills, California. She was awarded a B.A. in physiology in 1948 from the University of California, Berkeley, and then went on to the University of Washington, attaining a Ph.D. in 1958 in biochemistry. Osborn's thesis examined the functions of the vitamins and enzymes whose action depended on folic acid. In 1957, she reported the mode of action of the folic acid antagonist methotrexate, which became a major chemotherapeutic agent, especially for leukemia.
In 1959, Osborn started a postdoctoral fellowship in the Department of Microbiology at the New York University School of Medicine and also moved into a new research area, the biosynthesis of lipopolysaccharide. Abundant on the surface of certain bacteria, lipopolysaccharide is responsible for major immunological reactions and for the bacteria's characteristic toxicity. Osborn's work led to a new understanding of a previously unknown mechanism of polysaccharide formation and helped to identify a potential target for the development of new antibiotics and chemotherapeutic agents. In 1961, Osborn was promoted to instructor at the School of Medicine and 1 year later became an assistant professor. She then joined the faculty of the Albert Einstein College of Medicine in 1963 and was eventually promoted to associate professor in 1966. In 1968, Osborn became professor of microbiology at the University of Connecticut Health Center School of Medicine, where she has been since 1980.
For her contributions to biochemistry, Osborn was accepted as a fellow of the American Academy of Arts and Sciences in 1977 and was elected to the National Academy of Sciences in 1978. Other major distinctions include having served as president of the American Society of Biological Chemists in 1981 and as president of the Federation of American Societies for Experimental Biology in 1982. She was an associate editor for the Journal of Biological Chemistry and served on the editorial boards of several journals, including Biochemistry, the Journal of Biological Chemistry, and the Annual Review of Biochemistry.