William Mather Lewis (March 24, 1878 – November 11, 1945) was an American teacher, university president, local politician, and a state and national government official. He was mayor of Lake Forest, Illinois from 1915–17, President of George Washington University from 1923-7 and the President of Layfayette College from 1927-45.
Lewis received an A.B. from Lake Forest College in 1900, and an A.M. from Illinois College in 1902. Later, he would receive his Ph.D. from the University of Berlin. He was a member of Phi Delta Theta.
Lewis was briefly principal of Whipple Academy, Jacksonville (a preparatory school of Illinois College), before returning to Lake Forest to be head of the department of oratory and debate at Lake Forest Academy for three years. In 1905 he became headmaster at the academy, resigning in 1913 to travel and study in Europe. He was mayor of Lake Forest, Illinois from 1915–17.
Lewis was field secretary of the Navy League of the United States in the Mid-West in 1915.
During World War I, he was executive secretary of the National Committee of Patriotic Societies. Lewis was director of the savings division of the United States Treasury Department and chief of educational service for the U.S. Chamber of Commerce from 1921-23.
Because of his earlier work during World War I, Lewis was asked to become the director of the Pennsylvania Selective Service System (organising "the draft", which he did from September 1940 until he stepped down in November 1941 since it detracted from his duties as president of Lafayette College.