Reagan's widow, Nancy, said she was terribly saddened by the loss of a dear and trusted friend.
"Ronnie and Margaret were political soul mates, committed to freedom and resolved to end Communism.
"As prime minister, Margaret had the clear vision and strong determination to stand up for her beliefs at a time when so many were afraid to 'rock the boat.' As a result, she helped to bring about the collapse of the Soviet Union and the liberation of millions of people."
Former President Bill Clinton said Thatcher "understood that the special relationship which has long united our two nations is an indispensable foundation for peace and prosperity.
"Our strong partnership today is part of her legacy."
'Great hurt to Irish and British people'
But Northern Ireland politician and Sinn Fein leader Gerry Adams gave a very different view of Thatcher's legacy.
"Margaret Thatcher did great hurt to the Irish and British people during her time as British prime minister," he said in a statement.
"Working class communities were devastated in Britain because of her policies.
"Her role in international affairs was equally belligerent whether in support of the Chilean dictator (Agusto) Pinochet, her opposition to sanctions against apartheid South Africa; and her support for the Khmer Rouge.
"Here in Ireland, her espousal of old draconian militaristic policies prolonged the war and caused great suffering."
She will be remembered in particular for "her shameful role during the epic hunger strikes of 1980 and 81" and her Irish policy "failed miserably," he concluded.
His longtime political opponent in Northern Ireland, the former Rev. Ian Paisley, now Lord Bannside, had kinder words for Thatcher.
"In every phase of life she was great -- great as a woman, great as a wife, great as a mother, great as a political candidate, great as a Member of Parliament, especially as the first woman prime minister, great as a winner of the war, and great as a member of the House of Lords," he said.
Irish Prime Minister Enda Kenny also gave a more positive assessment.
"While her period of office came at a challenging time for British-Irish relations, when the violent conflict in Northern Ireland was at its peak, Mrs Thatcher signed the Anglo-Irish Agreement which laid the foundation for improved North-South cooperation and ultimately the Good Friday Agreement."
Falklands: 'We will always be thankful'
Officials in the Falkland Islands said they would never forget Thatcher's decision to defend the South Atlantic territory in 1982.
"Her friendship and support will be sorely missed, and we will always be thankful for all that she did for us," said Mike Summers of the Falkland Islands Legislative Assembly.
The United Kingdom and Argentina went to war over the territory in 1982 after the then-military government in Argentina landed troops on the islands. Argentina put its death toll from the conflict at around 645. Britain says its civil and military losses amounted to 255.