In his early years at MCG Greenblatt worked with Edgar Pund on the pathology and therapy of granuloma inguinale, a widely endemic venereal disease.
He helped develop mass production of penicillin for battlefield use; commanded a triage unit on the Okinawa beachhead for wounded Marines; and was among the first scientists to inspect the medical effects of the atomic bomb in Nagasaki.
Greenblatt started his clinical work in reproductive endocrinology, the branch of medicine concerned with infertility in women, when the field was in its infancy.
His major advances in the field include, in 1950, showing the effectiveness of estrogens in managing menopause symptoms and developing, in 1966, a monthly oral contraceptive pill, an accomplishment for which MCG received national attention.
In 1987, he authored Sex and Circumstance: Humanity in History, which contained 44 vignettes detailing the sexual nature of people ranging from U.S. President John F. Kennedy to Pyotr Ilyich Tchaikovsky.