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 and 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.