She was elected leader of the Conservative Party in 1975, when the party was in opposition.
She made history four years later, becoming prime minister when the Conservatives won the elections of 1979, the first of three election victories to which she led her party.
As British leader, Thatcher took a firm stance with the European Community -- the forerunner of the European Union -- demanding a rebate of money London contributed to Brussels.
Her positions on other issues, both domestic and foreign, were just as firm, and in one of her most famous phrases, she declared at a Conservative Party conference that she had no intention of changing her mind.
"To those waiting with bated breath for that favorite media catchphrase, the U-turn, I have only one thing to say: 'You turn if you want to. The lady's not for turning,'" she declared, to cheers from party members.
The United Kingdom fought a short, sharp war against Argentina over the Falklands Islands under Thatcher in 1982, responding with force when Buenos Aires laid claim to the islands.
Announcing that Britain had recaptured South Georgia Island from Argentina, Thatcher appealed to nationalist sentiments, advising the press: "Just rejoice at the news and congratulate our forces."
A journalist shouted a question at her as she turned to go back into 10 Downing Street: "Are we going to war with Argentina, Mrs. Thatcher?"
She paused for an instant, then offered a single word: "Rejoice."
Controversy over Falklands war
The conflict was not without controversy, even in Britain.
A British submarine sank Argentina's only cruiser, the General Belgrano, in an encounter that left 358 Argentines dead. The sinking took place outside of Britain's declared exclusion zone.
In her first term, Thatcher reduced or eliminated many government subsidies to business, a move that led to a sharp rise in unemployment. By 1986, unemployment had reached 3 million.
But Thatcher won landslide re-election in 1983 on the heels of the Falklands victory, her Conservative Party taking a majority of seats in parliament with 42% of the vote. Second-place Labour took nearly 28%, while the alliance that became the Liberal Democrats took just over 25%.
A year later, she escaped an IRA terrorist bombing at her hotel at the Conservative Party conference in Brighton.
She was re-elected in 1987 with a slightly reduced majority.
She was ultimately brought down, not by British voters, but by her own Conservative party.
Brought down by the poll tax
She was forced to resign in 1990 during an internal leadership struggle after she introduced a poll tax levied on community residents rather than property.
The unpopular tax led to rioting in the streets.