Thatcher, Margaret (Hilda), Baroness Thatcher of Kesteven (b.1925), British Conservative stateswoman, Prime Minister 1979-90. She was the country's first woman Prime Minister, and became the longest-serving British Prime Minister of the 20th century. Her period in office was marked by an emphasis on monetarist policies, privatization of nationalized industries, and trade union legislation. She became known for her determination and her emphasis on individual responsibility and enterprise.