Architectural historian of classical Rome, professor of art, Smith College, 1965- . MacDonald's father was William Lloyd MacDonald, Sr., and his mother Susan E. Elrod (MacDonald). He served in World War II in the U.S. Army Air Forces, 1942-1945, rising to first lieutenant. Afterward, MacDonald entered Harvard University, receiving his A.B. in 1949. He began lecturing at:Boston Architectural Center in the history of architecture in 1950, continuing for his A.M.at Harvard. During that time he participated in the excavation of the mosaics at Hagia Sophia, organized by the Byzantine Institute, beginning in 1951, immediately after the death of its instigator, Thomas Whittemore. His master's degree was awarded from Harvard in 1953. The same year he married Dale Ely. MacDonald taught at Wheaton College, Norton, MA, as an instructor of classics in 1953 while pursuing his Ph.D. After an American Academy in Rome fellowship, 1954-1956, he graduated in 1956, writing his dissertation on the Hippodrome structure in Byzantium. The same year he joined Yale University as an instructor, promoted to assistant professor in 1959. He published Early Christian and Byzantine Architecture in 1962 and the following year was made associate professor of history of art at Yale. His second book, The Architecture of the Roman Empire appeared in 1965. In what has been described as a "generational transition/bloodbath" in the department (Sears), MacDonald left along with several other younger scholars, including Spiro Kostof. He joined Smith College, Northampton, MA, as professor of art in 1965 where he remained the rest of his career. Macdonald was appointed A. P. Brown Professor in 1974. He wrote several moving biographical entries on Whittemore, an important man-of-action for Byzantine art who had heretofore not been documented, beginning with a piece for the American Biographical Dictionary in 1974. He also authored the book, Northampton Massachusetts Architecture and Buildings, 1975. His Architecture of the Roman Empire was reissued as a 3-volume paperback set in the 1980s, part of the Yale Publications in the History of Art. It became the standard text for undergraduate courses for most of the second half of the twentieth century.

Examines Roman architecture as a party of overall urban design and looks at arches, public buildings, tombs, columns, stairs, plazas, and streets.