In that post, she earned the unwanted nickname "Thatcher the milk snatcher" because of her reduction of school milk programs. It was a taste of battles to come.
As prime minister, she sold off one state industry after another: British Telecom, British Gas, Rolls-Royce, British Airways, British Coal, British Steel, the water companies and the electricity distribution system among them. She was proud of her government's role in privatizing some public housing, turning tenants into homeowners.
She ruffled feathers simply by being herself. She had faith - sometimes blind faith - in the clarity of her vision and little use for those of a more cautious mien.
Success in the Falklands War set the stage for a pivotal fight with the National Union of Miners, which began a 51-week strike in March 1984 to oppose the government's plans to close a number of mines.
The miners battled police on picket lines but couldn't beat Thatcher, and returned to work without gaining any concessions.
She survived an audacious 1984 assassination attempt by the Irish Republican Army that nearly succeeded. The IRA detonated a bomb in her hotel in Brighton during a party conference, killing and injuring senior government figures, but leaving the prime minister and her husband unharmed.
Thatcher won a third term in another landslide in 1987, but may have become overconfident.
She trampled over cautionary advice from her own ministers in 1989 and 1990 by imposing a hugely controversial "community charge" tax that was quickly dubbed a "poll tax" by opponents. It was designed to move Britain away from a property tax and instead imposed a flat rate tax on every adult except for retirees and people who were registered unemployed.
That decision may have been a sign that hubris was undermining Thatcher's political acumen. Tens of thousands of protesters took to the streets in London and other cities, leading to some of the worst riots in the British capital for more than a century.
The shocking sight of Trafalgar Square turned into a smoldering battleground on March 31, 1990, helped convince many Conservative figures that Thatcher had stayed too long.
"How could a leader who was wise make 13 million people pay a tax they had never paid before? It just showed that she was no longer thinking in a rational way," one of her junior ministers, David Mellor, said in a BBC documentary.
For Conservatives in Parliament, it was a question of survival. They feared vengeful voters would turn them out of office at the next election, and for many that fear trumped any gratitude they might have felt for their longtime leader.
Eight months after the riots, Thatcher was gone, struggling to hold back tears as she left Downing Street after being ousted by her own party.
It was a bitter end for Thatcher's active political career - her family said she felt a keen sense of betrayal even years later.
In 1992, she was appointed in the House of Lords, taking the title Baroness Thatcher of Kesteven.
Thatcher wrote several best-selling memoirs after leaving office and was a frequent speaker on the international circuit before she suffered several small strokes that in 2002 led her to curtail her lucrative public speaking career.
Denis Thatcher died the following year; they had been married more than a half century.
Thatcher's later years were marred by her son Mark Thatcher's murky involvement in bankrolling a 2004 coup in Equatorial Guinea. He was fined and received a suspended sentence for his role in the tawdry affair.
She suffered from dementia in her final years, and her public appearances became increasingly rare.
She is survived by her two children, Mark Thatcher and Carol Thatcher, and her grandchildren.