Stuart was born in Chicago in 1886, the son of Robert Stuart and his wife, Margaret. In 1899, Stuart's father and his business partner, Henry Parsons Crowell, used a proxy fight to gain control of the American Cereal Company from Ferdinand Schumacher. In 1901, they renamed the company the Quaker Oats Company to take advantage of the brand that had been built up by Crowell's Quaker Mill Company. Stuart's father was thus one of the co-founders of the Quaker Oats Company.
From 1901 to 1921, Robert Stuart had been responsible for managing the Quaker Oats Company's facilities, and Crowell had been responsible for marketing. Douglas Stuart's older brother, John Stuart, was groomed to take over Robert Stuart's responsibilities, while Douglas Stuart trained under Crowell. In 1922, the elder Stuart and Crowell retired, and John and Douglas Stuart took over the day-to-day operations of the company.
Stuart was active in the Republican. His son, R. Douglas Stuart, Jr., was the founder of the America First Committee, which opposed United States participation in World War II, in September 1940.
After World War II, Stuart took Quaker Oats' marketing in a bold new direction, with Quaker Oats becoming one of the first companies to employ the services of more than one advertising agency.
Besides Quaker Oats, Stuart was active in the community. TheHe was a delegate to the 1952 Republican National Convention. He was also long-time president of the Chicago Council of the Boy Scouts of America.
In 1953, President of the United States Dwight Eisenhower named Stuart United States Ambassador to Canada. Stuart presented his credentials to Vincent Massey, Governor General of Canada, on July 15, 1953. He served in Ottawa until May 4, 1956.
After serving as ambassador, Stuart returned to Quaker Oats. He retired as chairman of the board in 1962.
In addition to his son, Stuart and his wife had three daughters, one, Anne, who would go on to marry Nebraska politician Clifton Batchelder.
Stuart died at his home in Lake Forest, Illinois on January 5, 1975.