Sarah Stewart was born in Mexico to an American father and Mexican mother. She moved with her family back to the United States at age 5. She graduated from New Mexico State University in 1927.

Stewart earned her MS in Microbiology from University of Massachusetts-Amherst in 1930 and her Ph.D. in Microbiology from the University of Chicago in 1939. She later became the first woman to receive an M.D. from Georgetown University in 1949 (at the age of 43). 

She worked at the National Institutes of Health (NIH) while also completing her Ph.D., but her request to study the link between viruses and cancer was denied. The National Cancer Institute at the NIH cited a lack of education and experience with human research as the reason for their refusal to fund her. Until the 1960s, most scientists considered the idea of a cancer-causing virus to be preposterous. She left the NIH in 1944 to teach at Georgetown University in the medical school. While teaching, she was able to take medical courses until she was able to officially enroll in medical school when Georgetown began accepting women in 1947.

The NIH continued to deny Dr. Stewart’s funding to study cancer, so she took a temporary position in gynecology at a hospital in Staten Island to gain more experience. Finally, after an appointment in the United States Public Health Service (PHS) Commissioned Corps and a position at the National Cancer Institute (NCI) in Baltimore, the NIH accepted her request to study cancer. She returned to the NIH in 1951 to work at their NCI. Dr. Stewart was the Head of the Human Virus Studies Section in the Laboratory of Viral Oncology at the NCI at the NIH.

In 1956 she isolated the SE (Stewart-Eddy) polyoma virus with Dr. Bernice Eddy of the Division of Biologics Standards (DBS) Laboratory of Virology & Rickettsiology. The SE polyoma virus induces parotid gland tumors and a variety of other primary neoplasms in mice and other animals, which had implications for future viral oncology research. . Without Drs. Stewart and Eddy’s persistence on the ability of viruses to cause cancer, the human papillomavirus (HPV) vaccine would not exist today.