During his time in medical school Wiener did research work on blood groups at the Jewish Hospital of Brooklyn and from 1930 to 1932 he interned there and kept up a lifelong affiliation with that institution as the head of the Division of Genetics and Biometrics (1933–1935) and as the head of the blood transfusion division until 1952. Since 1932 he had a medical practice and in 1935 he founded the "Wiener Laboratories" for clinical pathology and blood grouping. In 1938 he became a member of the Department of Forensic Medicine of New York University School of Medicine, rising to the rank of professor by 1968. Since the 1930s he co-operated with the office of the Chief Medical Examiner of New York City.