Hattie Alexander was born in Baltimore, Maryland on April 5, 1901. She attended Goucher College in Baltimore (A.B., 1923), worked as a bacteriologist for the United States and Maryland Public Health Services for three years, and then attended Johns Hopkins Medical School (M.D., 1930). After interning at the Harriet Lane Home of the Johns Hopkins Hospital for a year, Alexander continued her internship at Babies Hospital at the Columbia-Presbyterian Medical Center in 1931. She remained there for the rest of her career, and rose through a succession of academic appointments at Columbia University's College of Physicians and Surgeons (Holt Fellowship in Diseases of Children, 1932-34; Assistant in Diseases of Children, 1933-35; Instructor in Diseases of Children, 1935-36; Associate in Pediatrics, 1936-43; Assistant Professor of Pediatrics, 1943-48; Associate Professor of Pediatrics, 1948-58; Professor of Pediatrics, 1958-66; Emeritus Professor of Pediatrics, 1966-68) and clinical appointments at Babies Hospital and Vanderbilt Clinic (Assistant Attending Physician, 1933-39; Assistant Attending Pediatrician, 1939-42; Associate Attending Pediatrician, 1942-51; Attending Pediatrician, 1951-66; Consultant, 1966-68).
At Babies Hospital, Alexander headed the microbiological laboratory and became a leading authority on bacterial infections. In 1939, she gained international recognition by developing a rabbit anti-serum that was the first effective treatment of influenzal meningitis in infants, a previously fatal disease. She also conducted research on the mechanisms of bacterial resistance to antibiotics, and was an early researcher in microbiological genetics. In both her research and teaching, Alexander was known for her vigorous scholarly skepticism.
She belonged to many professional organizations, and in 1964 she became the first woman President of the American Pediatric Society. She was also a consultant to the Secretary of War's Influenza Commission from 1941 to 1945. She received numerous awards, including the E. Mead Johnson Award for Research in Pediatrics (1942), the Stevens Triennial Prize of the College of Physicians and Surgeons (1954), the Elizabeth Blackwell Award for Women in Medicine (1956), the Oscar B. Hunter Memorial Award of the American Therapeutic Society (1961), the Columbia-Presbyterian Medical Center Distinguished Service medal (1963), the Children's Hospital of Philadelphia medal (1965), the Heart Award of the Variety Club of Philadelphia (1966), and an honorary doctorate from Wheaton College (1967).
With Dr. Elizabeth Ufford, Alexander lived in Port Washington, New York, and enjoyed cultivating orchids, listening to music, and riding in her speedboat. Alexander died from cancer on June 24, 1968.