In what year did Margret thacher become the prime minister?

Margaret Thatcher became Britain's first female Prime Minister on 4 May 1979.

The Conservative party led by Margaret Thatcher won a parliamentary majority in the 1979 general election, and this was followed in 1983 by a landslide Conservative victory and re-election in 1987.

Thatcher retained power for eleven years, resigning in November 1990.