Born in the small market town of Grantham, in Lincolnshire, Margaret Hilda Roberts idolized her father, a grocer, who was also a staunch Methodist and a leader of the local community. From him and from her local grammar school she absorbed the Victorian values that were later to underpin her political programmes – self-reliance, self-improvement, thrift, and a strong sense of moral certainty.

At Oxford University she studied chemistry and became president of the University Conservative Association. After graduating, she worked as a research chemist in the plastics industry before studying law. She specialized in tax law and was called to the Bar in 1953. In 1951 she married Denis Thatcher, a wealthy businessman who loyally supported her political aspirations. They had twin children – Mark, who eventually married a Texan heiress, and Carol, who became a journalist and broadcaster. After unsuccessfully attempting to enter Parliament in 1950 and 1951, Margaret Thatcher was finally elected to the House of Commons in 1959, as Conservative MP for Finchley, north London.

After serving as a junior minister in the government of Harold Macmillan, Thatcher became a member of the cabinet of Edward Heath in 1970 as secretary of state for education. Her abolition of free school milk earned her the nickname “Ma Thatcher, Milk Snatcher” and gave her a reputation as a politician who was quite willing to be unpopular if she thought she was right. Following the fall of the Heath government after a bruising confrontation with the trade unions, Thatcher became one of the challengers to Heath's leadership of the party in 1975; against all expectations, she won on the first ballot. When James Callaghan's Labour government was brought down in 1979 by a “winter of discontent,” largely orchestrated by trade union leaders, Thatcher came to power as Britain's (and Europe's) first woman prime minister.

Her first aims were to reassert the authority of government, while making industry and public services more efficient by cutting their subsidies and forcing them to meet the disciplines of the market. She proclaimed herself a “conviction” politician, rather than a compromiser who would resolve conflicts by seeking consensus. Although “Thatcherism” was based as much on opportunism as on ideology, she did pursue a number of consistent aims – privatizing major government-owned enterprises, such as British Airways, the telephone system, and the service industries; curbing the powers of trade unions by legislation; fighting inflation; and encouraging popular capitalism by promoting share ownership and establishing the right of tenants in public housing to buy their homes instead of renting them.

By 1981 Thatcher's confrontational style and radical economic policies had raised unemployment to its highest level in half a century and provoked riots in deprived inner-city areas. Opinion polls showed her to be the most unpopular prime minister since 1945.

In 1982 Argentina's surprise invasion of the British-occupied Falkland Islands presented a direct external challenge that enabled Thatcher to make a comeback in political popularity. Accepting the grave risks involved in sending a hastily assembled task force 8,000 miles to face a numerically superior enemy in entrenched positions, she unhesitatingly opted to fight. This decision, vindicated by the professionalism of the armed forces, brought victory that confirmed her status as “the Iron Lady” and swept her to a landslide triumph at the polls in 1983.

Fortified by success, Thatcher prepared to take on the toughest union, that of the coal miners, which she defeated after a year-long strike (1984–85). Meanwhile, in 1984, she survived an assassination attempt by the Irish Republican Army, which blew up the hotel in which she and other cabinet members were staying during the Conservative Party conference at Brighton. Within half an hour of the blast she was calmly giving an interview to journalists. Although unemployment remained a major problem, for those in work incomes were rising fast, and in 1987 she defeated a still-fragmented opposition to win a third successive election, a unique achievement in modern British history. In 1990 Thatcher became the longest-serving prime minister since the 1820s. Her long tenure of office had by now made her a commanding figure in the international arena, a trusted confidante of Presidents Reagan and Bush, and a key figure in the politics of the European Community and post-communist eastern Europe.

Her fall from power came following a downturn in the economy, quarrels with senior colleagues over European policy, and the imposition of a deeply unpopular poll tax to fund local government, which was widely denounced as unjust and unworkable. Challenged for the leadership of the Conservative Party by Michael Heseltine in November 1990, she failed to win an outright victory in the first ballot and resigned when defeat in the second ballot appeared certain. Although overthrown by a party hierarchy that had become intolerant of her uncompromising attitudes, she remained the idol of many of the party faithful. Her successor, John Major, though more conciliatory in style, continued her policies of privatization and safeguarding national rights in the European Union.

Despite her removal from office, Thatcher could look on with satisfaction at the adoption of “Thatcherite” privatization programmes throughout most of the world and the acceptance by her lifelong opponents, Britain's Labour Party, of the superiority of the free market over state direction of the economy. In 1992 she was created Baroness Thatcher of Kesteven.

Her publications include the autobiographical The Downing Street Years 1979–1990 (1993) and The Path to Power (1995).