Younger died Sunday at the family compound in Scotland, former colleagues said. A spokesman for the Royal Bank of Scotland, where Younger was once chairman, said the cause of death was cancer.
Politicians paid tribute to the former lawmaker, who was made a life peer in 1992 to become Lord Younger of Prestwick, and subsequently inherited the hereditary title Lord Younger of Leckie on the death of his father in 1997.
"His life was one of committed and distinguished service to his country and his party," Thatcher said. "He was the embodiment of decency, honor and fair play in politics, and we shall all miss him greatly."
"George Younger was in the front line of British politics, bravely arguing for strong defense and the nuclear deterrent, and as a result he played an important part in the demise of the Soviet Union," said Tory leader Iain Duncan Smith.
George Kenneth Hotson Younger was born on Sept. 22, 1931, into an aristocratic Scottish family. He was educated at Winchester. Before studying history at New College, Oxford, he was a platoon commander during the Korean War.
He was elected to Parliament in the Scottish constituency of Ayr in 1964 and held the seat until he retired from the House of Commons in 1992.
Younger was secretary of state for Scotland from 1979 to 1986, and was defense secretary from 1986 to 1989. In that role, he strongly supported nuclear defense and said it would be "folly to dismantle or let decay" the North Atlantic Treaty Organization's deterrent weapons system.
Younger remained one of Thatcher's staunchest allies. He stepped down from politics in 1992 to pursue a business career. His chairmanship of the Royal Bank of Scotland from 1991 to 2001 coincided with the most successful decade in its history.
He is survived by his wife, three sons and a daughter.