Margaret Thatcher's term as the Prime Minister of the United Kingdom began on 4 May 1979 when she accepted an invitation of Queen Elizabeth II to form a government, and ended on 28 November 1990 upon her resignation. She was elected to the position in 1979, having led the Conservative Party since 1975, and won landslide re-elections in 1983 and 1987. She gained intense media attention as Britain's first female prime minister, and was the longest-serving British prime minister of the 20th century. Her premiership ended when she withdrew from the 1990 Conservative leadership election. While serving as prime minister, Thatcher also served as the First Lord of the Treasury, the Minister for the Civil Service and the Leader of the Conservative Party.

The 1990 Conservative Party leadership election was called on 14 November 1990 following the decision of Michael Heseltine, former defence and environment secretary, to challenge Margaret Thatcher, the incumbent Prime Minister of the United Kingdom, for leadership of the Conservative Party.