John Llewellyn Lewis was an American leader of organized labor who served as president of the United Mine Workers of America (UMW) from 1920 to 1960. A major player in the history of coal mining, he was the driving force behind the founding of the Congress of Industrial Organizations (CIO), which established the United Steel Workers of America and helped organize millions of other industrial workers in the 1930s, during the Great Depression. After resigning as head of the CIO in 1941, thus keeping his promise of resignation if President Franklin Delano Roosevelt won the 1940 election against Wendell Willkie, Lewis took the United Mine Workers out of the CIO in 1942 and in 1944 took the union into the American Federation of Labor (AFL).

Labor unions represent United States workers in many industries recognized under US labor law since the 1935 enactment of the National Labor Relations Act. Their activity today centers on collective bargaining over wages, benefits, and working conditions for their membership, and on representing their members in disputes with management over violations of contract provisions. Larger labor unions also typically engage in lobbying activities and electioneering at the state and federal level.