She was the longest serving British Prime Minister in the 20th century, the longest since Gladstone, and had the longest single period in office since Lord Liverpool. She is also the only woman to be UK Prime Minister or leader of a major British political party. Undoubtedly one of the most significant British politicians in recent political history, she was both loved and loathed by many people.

She was also Secretary of State for Education and Science from 1970 to 1974, and Leader of the Opposition from 1975 to 1979. She won three successive general elections as party leader, the only British politician to do so in the 20th century. However, although she had strong support from the largest minority of voters for most of her tenure she eventually resigned after failing to win outright a leadership election triggered by opponents within her own party, and was replaced by John Major in 1990. She is an elder stateswoman of the Conservative Party and the figurehead of a political philosophy that became known as Thatcherism, which involves selectively reduced public spending in some areas, lower direct taxation, de-regulation, a monetarist policy, and a programme of privatisation of government-owned industries. Even before coming to power she was nicknamed the "Iron Lady" in Soviet media (because of her vocal opposition to communism), an appellation that stuck.

Thatcher served as Secretary of State for Education and Science in the government of Edward Heath from 1970 to 1974, and successfully challenged Heath for the Conservative leadership in 1975. As party Leader she was undefeated at the polls, winning the 1979, 1983 and 1987 general elections. In foreign relations, she maintained the "special relationship" with the United States, and formed a close bond with Ronald Reagan. In 1982 her government dispatched a Royal Navy task force to retake the Falkland Islands from Argentina in the Falklands War.

The profound changes Thatcher set in motion as Prime Minister altered much of the economic and cultural landscape of the United Kingdom. She curtailed the power of the trade unions, attempted to cut back the role of the state in business, and dramatically expanded home ownership, all of which were intended to create a more entrepreneurial culture. She also aimed to cut back the welfare state and foster a more flexible labour market which she believed would create jobs and could adapt to market conditions. Exacerbated by the global recession of the early 1980s, her policies caused large-scale unemployment and contributed to the continued 'de-industrialisation' of the UK. She is particularly disliked in the old mining areas of Britain, such as South Wales, County Durham and the southern parts of Yorkshire. In a speech on 19th June, 1984, she had referred to the striking miners as "the enemy within", who had to be defeated for the good of liberty and democracy.

Margaret Thatcher and her policies, known as Thatcherism were, and remain, highly controversial and polarising. Her supporters contend that she was responsible for rejuvenating the British economy, while her opponents argue that she was responsible for mass unemployment and a vast increase in inequality between rich and poor. Some have since argued that the hardships and disruption of the period were a regrettable but necessary phase in the modernisation of the British economy, but the perception that her Conservative party was unconcerned or blind to these effects has contributed to the subsequent dominance of the Labour Party in government. Both Conservative and Labour governments since 1990 have maintained most of the economic reforms of the Thatcher period; outside observers argue over the degree to which these reforms or the increased public spending of the Blair governments are responsible for the recent continued and stable growth of the economy.