The longest-serving Speaker of the U.S. House of Representatives was Sam Rayburn, who served from 1940 to 1961. His tenure lasted a total of 17 years and was marked by significant influence over legislative matters and leadership within the Democratic Party. Rayburn played a crucial role in shaping major policies, including the New Deal and post-World War II legislation. His ability to foster bipartisan support and maintain party unity contributed to his lasting legacy in American politics, highlighting the importance of effective leadership in the legislative process.