William M. Colmer (February 11, 1890 - September 9, 1980) was a Mississippi politician.
Colmer was born in Moss Point, Mississippi. Colmer attended Millsaps College. He served in the military during World War I.
Colmer was elected Jackson County attorney in 1921, becoming district attorney in 1928.
In 1932, Colmer was elected to the House of Representatives as a Democrat from Mississippi's 6th District, located on the Gulf Coast. He was reelected 19 times. His district was renumbered the 5th after the 1960 Census, when Mississippi's declining population cost it a congressional seat.
Originally elected as a supporter of the New Deal, Colmer became increasingly conservative as the years passed, especially as the Democrats became more friendly towards civil rights. After the Brown v. Board of Education decision, Colmer helped to get Southern congressmen to sign the Southern Manifesto. He endorsed Richard Nixon for president in 1960, 1968 and 1972 and Barry Goldwater in 1964. Despite his strong opposition to racial integration, he served as the chairman of the Rules Committee from 1967 to 1973.
Colmer did not run for reelection in 1972. He endorsed his administrative assistant, Trent Lott, as his successor even though Lott ran as a Republican. He served longer in either house of Congress than anyone in Mississippi's history except Jamie Whitten, who served 54 years in Congress.
Preceded by: Robert S. Hall United States Representative for the 6th Congressional District of Mississippi 1933-1963 Succeeded by: 6th district eliminated after Census 1960 Preceded by: W. Arthur Winstead United States Representative for the 5th Congressional District of Mississippi 1963-1973 Succeeded by: Trent Lott