William Francis Mahar Sr. (January 1, 1919 – October 13, 2006) was an American politician.
Born in Belleville, Wisconsin, Mahar served in the United States Army during World War II and was a commissioned colonel. He participated in the Battle of the Bulge and was a Bronze Star recipient. He went to the University of Wisconsin, where he studied history and political science. He served on the Homewood, Illinois village board and as village president. Mahar served in the Illinois House of Representatives from 1973 to 1981 and was a Republican. Mahar then served in the Illinois Senate from 1981 to 1985. His son William F. Mahar Jr. also served in the Illinois General Assembly.