The record for the longest-serving Prime Minister in the U.K. is held by Margaret Thatcher. She served from May 4, 1979, to November 28, 1990, spanning over 11 years. As leader of the Conservative Party, Thatcher implemented significant economic and social reforms, famously known as "Thatcherism," which emphasized free-market policies, deregulation, and privatization of state-owned industries. Her tenure fundamentally shaped British politics and society, provoking both strong support and opposition. Thatcher's leadership style and policies left a lasting impact on the U.K. and the global political landscape.