Speaking to the BBC, former U.S. Secretary of State Henry A. Kissinger said Thatcher was a "great leader" and a "good friend of the United States."
She was known for her close working relationship with President Ronald Reagan.
"It is with great sadness that Mark and Carol Thatcher announced that their mother, Baroness Thatcher, died peacefully following a stroke this morning," her spokesman, Tim Bell, said in a statement.
Within moments of the announcement, Queen Elizabeth II and Prime Minister David Cameron offered tributes to the woman Cameron called "a great leader, a great prime minister, a great Briton," The New York Times reported.
"She didn't just lead our country, she saved our country," said Cameron, who returned to Britain early from a visit to Spain.
U.S. President Obama said in a statement the world "lost one of the great champions of freedom and liberty, and America has lost a true friend."
Noting her beginnings as a grocer's daughter, Obama said Thatcher was "an example to our daughters that there is no glass ceiling that can't be shattered."
"As prime minister, she helped restore the confidence and pride that has always been the hallmark of Britain at its best," he said. "And as an unapologetic supporter of our trans-Atlantic alliance, she knew that with strength and resolve we could win the Cold War and extend freedom's promise."
He said he and first lady Michelle Obama sent their thoughts to the Thatcher family and all Britons "as we carry on the work to which she dedicated her life -- free peoples standing together, determined to write our own destiny."
Thatcher, known as the "Iron Lady," served as prime minister for 11 years, beginning in 1979. She was elected three times.
Buckingham Palace said the queen was "sad to hear the news" and would send a private message of condolence to the family.
U.S. Secretary-General Ban Ki-moon called Thatcher a "pioneering lady" who contributed to peace and security, especially at the height of the Cold War.
"We will miss her great leadership," Ban said in a statement.
"She was also a great model as the first woman prime minister of the United Kingdom, who not only demonstrated her leadership but has given such great hope for many women for equality, gender equality in Parliament," Ban said.
The last time Thatcher addressed the U.N. General Assembly as prime minister was November 1989 when she focused on the global environment and the need to preserve air, water and land.
U.S. Secretary of State John Kerry said the United States lost a friend and "the world has lost a transformative leader who broke the glass ceiling in global politics."
"With her characteristic determination, she defined grit on the world stage and grace in the face of advancing age and illness," Kerry said in a statement.
"Lady Thatcher took the helm of government amid tumultuous times," he said. "She would face wars abroad, terrorism at home and deep uncertainty about the United Kingdom's future. She met all these challenges and many others with unyielding drive and courage."
Lawmakers said Thatcher, who had been in poor health for months, retired from public life about two years ago. Her career in active politics ended in 1990 when her Conservative Party forced her from office and replaced her with John Major.