[1] In 1920, during a period of political and economic turmoil following World War I, his family moved to the United States.

After graduating from high school he took up a job as a shoe salesman, but later chose to study chemistry at New York University.

[4] Trained in New Orleans, he served in administrative roles at military hospitals in Fort Leavenworth, Kansas and, later, near Karachi.

[4] He began working as a clinical instructor at the Marquette University School of Medicine in 1948 and taught there until 1966, eventually becoming full professor.

[3][5] He established a rare donor registry at the AABB in 1959,[3] and published papers on the management of blood banks.

[6] In 1967, Greenwalt relocated to Washington, D.C. to serve as the national director of the American Red Cross's blood program, where he worked to establish another rare donor registry.

[2] He was made an emeritus professor of internal medicine and pathology at the University of Cincinnati in 2003,[1] and continued working in this role until his death at the age of 91.