William B. Kountz was born in Saxton, Missouri in 1896 and attended schools in nearby St. Joseph. He entered Washington University as an undergraduate in 1918 and (without formally completing a bachelor's degree) continued on through medical school, graduating in 1926. Until 1928, Kountz was an intern and resident physician at Barnes, St. Louis Maternity, and St. Louis Children's Hospitals. He then became a physician with the cardiovascular service of the Washington University Dispensary. The award of a national Research Council Fellowship offered Kountz the opportunity for further training abroad, and for eighteen months beginning in 1930 he visited hospitals in Britain, Germany, and Egypt. (He later recalled that the chance to study Egyptian mummies inspired his interest in aging.) He returned in late 1931 to join the clinical faculty of the School and to launch he career as a leading cardiologist in St. Louis.
Kountz served on the staff of Barnes and Lutheran Hospitals, but became particularly well known for his treatment and study of aged patients at the St. Louis City Infirmary (later, St. Louis Chronic Hospital) on Arsenal Street. In 1938 he helped establish a special geriatrics research unit at the Infirmary. In 1946 WUSM organized a Division of Gerontology based at the Chronic Hospital and Kountz was named its director of clinical services. That same year Kountz became a founding member of the American Society for the Study of Arteriosclerosis, which later became a branch of the American Heart Association. In 1954 Kountz's work received additional support through the establishment of a St. Louis-based Gerontological Research Foundation.
Kountz died in 1962. He was succeeded as head of the Division of Gerontology by John Esben Kirk, under whom the program continued until 1973.