The longest-serving British Prime Minister of the 20th century was Margaret Thatcher, who held office from 1979 to 1990. As the leader of the Conservative Party, she implemented significant political and economic reforms widely associated with Thatcherism, which emphasized deregulation, privatization of state-owned industries, and reducing the power of trade unions. Her leadership style was characterized by strong conviction and determination, making her a polarizing figure in British politics. Thatcher remains a seminal figure, known for her influence on both the UK and global politics during her time in power.