William MacDonough Agar (1894-1972) was an American geologist, educator, and author.

William McDonough Agar was born on February 14, 1894, in New York, New York, to parents Agnes Louise MacDonough and John Giraud Agar. In his youth, Agar attended the Newman School in Lakewood, New Jersey. He married Alida Steward Carter, with whom he had four children. He was a founder and board chairman of Freedom House, a research institute, in New York. Agar served as chief of visitors' services in the department of public information of the United Nations Secretariat from 1948 to 1956. He received a B.S. degree from Princeton in 1916, an M.A. in 1920, and a Ph. D. in 1922. From 1922 to 1923 he was a geologist working for Anaconda Copper Company in Butte, Montana. He taught geology at Yale from 1923 to 1928, and at Columbia from 1928 to 1935. Later, he became headmaster of the Newman School from which he had graduated, and was appointed chief geologist of the United States Geological Survey. Agar passed away on June 10, 1972 in Southampton, New York.