Margaret Thatcher served as Prime Minister of the United Kingdom from May of 1979 to November of 1990. She served as Prime Minister longer than anyone dating back to William Ewart Gladstone in the mid to late 1800's. She had the longest continuous period in office since Lord Liverpool in the early 1800's. She was one of two women ever to hold one of the four great offices of state. Margaret Thatcher was first elected prime minister in 1979. She was reelected in 1983 and 1987. She was the first and only woman prime minister in the United Kingdom's history. In 1990, she left office after over 11 years of service to the United Kingdom.