Margaret Thatcher was a person.
Margaret Hilda Thatcher , Baroness Thatcher , ( ; 13 October 1925 -- 8 April 2013 ) was a British stateswoman , who served as Prime Minister of the United Kingdom from 1979 to 1990 and as 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 have held the office . Edward Heath appointed her Secretary of State for Education and Science in his 1970 government . In 1975 , Thatcher defeated Heath in the Conservative Party leadership election to become Leader of the Opposition and became the first woman to lead a major political party in the United Kingdom . She became Prime Minister after winning the 1979 general election .