Born in Grantham in Lincolnshire, United Kingdom, she went to school at Kesteven and Grantham Girls' School in Grantham, where she was head girl in 1942–43. She read chemistry at Somerville College, Oxford and later trained as a barrister. She won a seat in the 1959 general election, becoming the MP for Finchley as a Conservative. When Edward Heath formed a government in 1970, he appointed Thatcher Secretary of State for Education and Science. Four years later, she backed Keith Joseph in his bid to become Conservative Party leader but he was forced to drop out of the election. In 1975 Thatcher entered the contest herself and became leader of the Conservative Party. At the 1979 general election she became Britain's first female Prime Minister.

In her foreword to the 1979 Conservative manifesto, Thatcher had written of "a feeling of helplessness, that a once great nation has somehow fallen behind." She entered 10 Downing Street determined to reverse what she perceived as a precipitate national decline. Her political philosophy and economic policies emphasised deregulation, particularly of the financial sector, flexible labour markets, and the selling off and closing down of state owned companies and withdrawing subsidy to others. Amid a recession and high unemployment, Thatcher's popularity declined, though economic recovery and the 1982 Falklands War brought a resurgence of support and she was re-elected in 1983. She took a hard line against trade unions, survived the Brighton hotel bombing assassination attempt and opposed the Soviet Union (her tough-talking rhetoric gained her the nickname the "Iron Lady"); she was re-elected for an unprecedented third term in 1987. The following years would prove difficult, as her Poll tax plan was largely unpopular, and her views regarding the European Community were not shared by others in her Cabinet. She resigned as Prime Minister in November 1990 after Michael Heseltine's challenge to her leadership of the Conservative Party.

Thatcher's tenure as Prime Minister was the longest since that of Lord Salisbury and the longest continuous period in office since Lord Liverpool in the early 19th century. She was the first woman to lead a major political party in the United Kingdom, and the first of only four women to hold any of the four great offices of state. She holds a life peerage as Baroness Thatcher, of Kesteven in the County of Lincolnshire, which entitles her to sit in the House of Lords.

Margaret Hilda Roberts was born on 13 October 1925 to Alfred Roberts, originally from Northamptonshire, and his wife, the former Beatrice Ethel Stephenson from Lincolnshire. Thatcher spent her childhood in the town of Grantham in Lincolnshire, where her father owned two grocery shops. She and her older sister Muriel (born 1921, Grantham; died December 2004; married name Cullen) were raised in the flat above the larger of the two located near the railway line. Her father was active in local politics and religion, serving as an Alderman and Methodist lay preacher. He came from a Liberal family but stood—as was then customary in local government—as an Independent. He lost his post as Alderman in 1952 after the Labour Party won its first majority on Grantham Council in 1950.

Margaret Roberts was brought up a strict Methodist by her father. Having attended Huntingtower Road Primary School, she won a scholarship to Kesteven and Grantham Girls' School. Her school reports show hard work and commitment, but not brilliance. Outside the classroom she played hockey and also enjoyed swimming and walking. Finishing school during the Second World War, she applied for a scholarship to attend Somerville College, Oxford, but was only successful when the winning candidate dropped out. She went to Oxford in 1943 and studied Natural Sciences, specialising in Chemistry. She became President of the Oxford University Conservative Association in 1946, the third woman to hold the post. At Oxford she read Friedrich von Hayek's recently published (1944) The Road to Serfdom. " I cannot claim that I fully grasped the implications of Hayek's little masterpiece at this time, [but] at this stage it was the..unanswerable criticisms of socialism in The Road to Serfdom which had an impact." In 1946 Roberts took the Final Honour School examination, graduating with a Second Class Bachelor of Arts degree. She subsequently studied crystallography and received a postgraduate BSc degree in 1947. Three years later, in 1950, she achieved a Master of Arts degree, according to her entitlement as an Oxford BA of seven years' standing since matriculation.

Following graduation, Roberts moved to Colchester in Essex, to work as a research chemist for BX Plastics. During this time she joined the local Conservative Association and attended the party conference at Llandudno in 1948, as a representative of the University Graduate Conservative Association. She was also a member of the Association of Scientific Workers. In January 1949, a friend from Oxford, who was working for the Dartford Conservative Association, told her that they were looking for candidates. After a brief period, she was selected as the Conservative candidate, and she subsequently moved to Dartford, Kent, to stand for election as a Member of Parliament. To support herself during this period, she went to work for J. Lyons and Co., where she helped develop methods for preserving ice cream and was paid £500 per year.

At the 1950 and 1951 elections, she fought the safe Labour seat of Dartford. Although she lost out to Norman Dodds, she reduced the Labour majority in the constituency by 6,000. She was, at the time, the youngest ever female Conservative candidate and her campaign attracted a higher than normal amount of media attention for a first time candidate. While active in the Conservative Party in Kent, she met Denis Thatcher, whom she married in 1951, conforming to his Anglicanism. Denis was a wealthy divorced businessman who ran his family's firm; he later became an executive in the oil industry. Denis funded his wife's studies for the Bar. She qualified as a barrister in 1953 and specialised in taxation. In the same year her twin children Carol and Mark were born, delivered by Caesarean section while their father watched a Test match at the Oval. With a mother climbing the political ladder, the children were left to a nanny. "My mother was prone to calling me by her secretaries' names and working through each of them until she got to Carol", recalled her daughter.

Thatcher began to look for a safe Conservative seat in the mid-1950s and was narrowly rejected as candidate for the Orpington by-election in 1955, and was not selected as a candidate in the 1955 election. She had several further rejections before being selected for Finchley in April 1958. She won the seat after hard campaigning during the 1959 election and was elected as a Member of Parliament (MP). Her maiden speech was in support of her Private Member's Bill (Public Bodies (Admission to Meetings) Act 1960) requiring local councils to hold meetings in public, which was successful. In 1961 she went against the Conservative Party's official position by voting for the restoration of birching.

Within two years, in October 1961, she was given a promotion to the front bench as Parliamentary Undersecretary at the Ministry of Pensions and National Insurance. She held this post throughout the administration of Harold Macmillan, until the Conservatives were removed from office in the 1964 election. When Sir Alec Douglas-Home stepped down, Thatcher voted for Edward Heath in the leadership election of 1965 over Reginald Maudling. She was promoted to the position of Conservative spokesman on Housing and Land; in this position, she advocated the Conservative policy of allowing tenants to buy their council houses. The policy would prove to be popular. She moved to the Shadow Treasury team in 1966. As Treasury spokesman, she opposed Labour's mandatory price and income controls, which she argued would produce contrary effects to those intended and distort the economy.

Thatcher established herself as a potent conference speaker at the Conservative Party Conference of 1966, with a strong attack on the high-tax policies of the Labour Government as being steps "not only towards Socialism, but towards Communism". She argued that lower taxes served as an incentive to hard work. Thatcher was one of few Conservative MPs to support Leo Abse's Bill to decriminalise male homosexuality and voted in favour of David Steel's Bill to legalise abortion, as well as a ban on hare coursing. She supported the retention of capital punishment and voted against the relaxation of divorce laws.