Percy's daughter, Sharon Rockefeller, announced in 2009 that he had Alzheimer's disease. His death was confirmed by the office of her husband, Sen. Jay Rockefeller (D-W.Va.).
But the silver-haired senator came to power when moderate Republicans were becoming unfashionable on Capitol Hill. He ended up backing former President Ford for the Republican nomination in 1976 rather than go for it himself.
After that his chances seemed to fade. He won one more term in 1978 but was narrowly defeated for reelection in 1984 by Democratic Rep. Paul Simon.
In a statement, Sen. Rockefeller praised his father-in-law's evenhanded political stances: "His insistence on a balanced perspective in his public life, (calling himself 'fervently moderate'), helped us understand it is both possible and preferable to live in a world without partisanship."
Percy's differences with conservative Republicans showed early on as he clashed with President Nixon, opposing two successive U.S. Supreme Court nominees — Clement F. Haynsworth and G. Harrold Carswell.
The engineer of a spectacular turnaround at camera maker Bell & Howell Co., Percy was an apostle of free markets who sought to ease federal regulation of U.S. corporations.
He often said that like Dwight D. Eisenhower he was "a conservative on money issues but a liberal on people issues."
Percy also opposed excessive partisanship, particularly as Foreign Relations Committee chairman.
"I don't want foreign policy developed just by one party" and then have to "ride roughshod over the other party," he told the Chicago Tribune in 1984. "I'd much more value a bill that has bipartisan support. That's what this committee achieved in World War II, achieved with the Marshall Plan."
Percy ran for governor of Illinois in 1964 but lost to Democratic incumbent Otto Kerner in an election year marked by a Democratic landslide.
Two years later, he unseated Democratic Sen. Paul Douglas, a classic New Deal liberal who had taught him economics at the University of Chicago in the 1930s.
A tragedy occurred in mid-campaign, and both candidates suspended campaigning for two weeks. One of Percy's 21-year-old twin daughters, Valerie, was bludgeoned and stabbed to death in her bed in the family's lakefront home in Kenilworth, a Chicago suburb. No one was ever charged in the case.
Sen. Dick Durbin (D-Ill.) said Saturday that he was with Douglas when he learned of Valerie Percy's death and that in the campaign's closing days, both candidates "showed a humanity and a respect which should be recalled in this era of venomous personal attacks and wild charges."
The surviving twin, Sharon Rockefeller, is president and chief executive of WETA, a public broadcasting station in Washington. Percy also had a son, Roger, with his first wife, Jeanne, who died in 1947 of a reaction to penicillin. He married Loraine Guyer in 1950, and they had a daughter, Gail, and a son, Mark.
Percy was elected when Illinois was a swing state where he could get votes from some Democrats and liberals. Early in his career, he had support from the United Auto Workers and always addressed the Illinois AFL-CIO labor union federation at campaign time.
The state gradually became more Democratic.
In 1978, he dug out from a deficit in the polls, looking squarely into the camera for TV ads to say: "I got the message." Percy squeaked in.
Defeated six years later, he remained in Washington and opened a foreign-policy consulting business.
Born in 1919 in Pensacola, Fla., he moved to Chicago with his family when he was very young. His family was poor for a time, and the young Percy took after-school jobs to help out.
His energy caught the eye of his Sunday school teacher, Joseph McNabb, chairman of Bell & Howell. Percy joined the firm after graduating from the University of Chicago in 1941.
After serving in the Navy for three years during World War II, Percy returned to Bell & Howell in 1945 and became chairman after McNabb died in 1949. Before Percy was through at the company, earnings were 32 times what they had been when he took over.