Theresa Scanlan was born in Decatur, Illinois on August 7, 1891. She entered the premedical program at the University of California in 1915, and from 1918 to 1920 she served in the Army Nurse Corps. She earned her M.D. at Physicians and Surgeons College of Columbia University in 1924. After her internship at Memorial Hospital in Worcester, Massachusetts, she became a resident at Bellevue Hospital, 1925-1926, where she began her specialty in tuberculosis. She opened a private practice, became a Fellow of the American College of Physicians, and taught medicine at New York Hospital of Cornell University. She was a member of the Women's Medical Association of New York City, the Women's Medical Society of New York State, and the American Medical Women's Association.