Born in Kassel, Germany in 1928, Hallo left Germany in 1939 for England, and immigrated to the United States with his family in 1941. He received his B.A. from Harvard and studied in the Netherlands through a Fulbright Fellowship at the University of Leiden. There he met and married Edith Pinto, with whom he had two children. After the death of Edith in 1991, he married Nanette Stahl.
Hallo studied for his Ph.D. 1951–1956 at the Oriental Institute with a fellowship from the University of Chicago under Professor I.J. Gelb. After receiving his Ph.D., he worked at Hebrew Union College’s Jewish Institute of Religion. In 1962, Hallo became assistant curator (and later curator) of the Babylonian Collection and professor of Assyriology at Yale, which he taught until his retirement in 2002.