Margaret Hilda Thatcher, Baroness Thatcher, (13 October 1925 – 8 April 2013) also known as the Iron Lady, was a British stateswoman who served as Prime Minister of the United Kingdom from 1979 to 1990 and leader of the Conservative Party from 1975 to 1990. She was the longest-serving British prime minister of the 20th century and the first woman to hold that office.