The last of the Stuart family line to have run Quaker Oats Co., and a co-founder of the America First Committee, which sought to keep the United States out of World War II,
Mr. Stuart became president in 1962, when his father retired as chairman, and introduced instant oatmeal in 1966, bought toy company Fisher-Price Co. in 1969 and launched Quaker Chewy Granola bars in 1981. He further diversified the oatmeal maker as CEO from 1966 to 1981.
The stage was set for the Gatorade-fueled transformation of Quaker with its acquisition of Gatorade owner Stokely-Van Camp Inc. in 1983, the year Mr. Stuart retired as chairman. A follow-on deal, the 1994 purchase of Snapple Beverage Corp., would prove disastrous.
The firm was founded in 1901 through a combination of four oat processors; one was a mill started in Iowa in the 1870s by Mr. Stuart's great-grandfather, Robert, and relative George Douglas. The Douglas family also gave its name to Chicago law firm Gardner Carton & Douglas, which before it was absorbed in a 2007 merger shared a Loop office tower with Quaker.
A Republican Party stalwart and U.S. ambassador to Norway from 1984 to 1989, Mr. Stuart was the sole surviving headliner of America First, to which he imparted a strong Chicago influence.
As a Yale University law student, he and future Yale President Kingman Brewster formed the group in 1940, recruiting Sears Roebuck & Co. chief Robert Wood to chair it and aviator Charles Lindbergh to add star power. Adherents on the Yale campus included Gerald Ford, Sargent Shriver and Potter Stewart, later an associate justice of the U.S. Supreme Court. John F. Kennedy sent a check from Harvard.
He signed up Midwestern executives Sterling Morton (of salt fame); Jay Hormel (meat), and William Regnery (textiles), whose son, Henry, went on to found the namesake publisher here.
Once the Japanese attack on Pearl Harbor ended the isolationist cause, and Mr. Stuart had volunteered for Army service in Europe, he returned to the family business.
In retirement, Mr. Stuart stayed active, funding Ronald Reagan's presidential campaigns, paving the way for an ambassadorship, and serving in the early 1990s on the Base Realignment and Closure Commission, which spared the Great Lakes Naval Training Station in his North Shore backyard.
A land partnership he co-founded in the 1950s became development company Conway Farms in Lake Forest.
Mr. Stuart’s 55-year marriage to Barbara Edwards ended with her death in 1993. In addition to his son Sandy, whose given name is Alexander, he is survived by his wife, Lillan Lovenskiold, whom he married in 1995; another son, James; a daughter, Marian Pillsbury; a sister, Margaret Stuart Hart; eight grandchildren; six great-grandchildren; and numerous stepchildren, stepgrandchildren and stepgreat-grandchildren.