John S. Monagan (born December 23, 1911) is a Connecticut politician and author.

After graduating from Dartmouth College and Harvard University law school, he served on the Board of Aldermen in Waterbury, Connecticut and became Mayor of Waterbury in 1943, serving until 1948. In 1958, he was elected Congressman from a district including Waterbury and served in the House of Representatives from 1959 until being defeated for re-election in 1972. After leaving Congress, Monagan practiced as a lawyer in Washington, DC and was active in amateur music and in charitable causes. Monagan has written several books including his memoirs and a biography of Justice Oliver Wendell Holmes, Jr. He also maintained a decades-long correspondence with the British novelist Anthony Powell.