Margaret Thatcher led a major political party in the United Kingdom.
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 . 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 resigned as Prime Minister and party leader in November 1990 , after Michael Heseltine launched a challenge to her leadership . The United Kingdom general election of 1979 was held on 3 May 1979 to elect 635 members to the British House of Commons . The Conservative Party , led by Margaret Thatcher , ousted the incumbent Labour government of James Callaghan with a parliamentary majority of 43 seats .