In 1955, he married Nancy Anderson. They had four children: Adlai Ewing 4th, Lucy, Katherine and Warwick. He is survived by his wife; sons Adlai and Warwick, daughters Lucy and Katherine and at least nine grandchildren, as well as his brothers John and Borden.
Mr. Stevenson graduated from Harvard Law School in 1957, and after serving as a law clerk for an Illinois Supreme Court justice in 1958, joined and later became a partner in a prestigious Chicago law firm, Mayer, Brown & Platt.
His political career began in 1964. Elected to a seat in the Illinois House of Representatives, he sponsored 84 bills in two years, including measures to control lobbying and conflicts of interest in government, a graduated state income tax and credit reforms. The Independent Voters of Illinois named him the house’s “best legislator.”
From 1967 to 1970, he was the elected Illinois state treasurer. In what might have been a routine post, he eliminated staff patronage, withdrew state funds from banks that practiced racial or religious discrimination, and put money in black-owned banks to finance small business, low-income housing and urban development.
After quitting active politics in 1986, Mr. Stevenson resumed practicing law, but in the 1990s he became president and chairman of SC&M Investment Management and later a co-founder of HuaMei Capital, both specializing in financial transactions between America and East Asia.
“The Stevensons: A Biography of an American Family,” by Jean H. Baker, appeared in 1996.
In 2008, Mr. Stevenson published “The Black Book,” a history, begun as a scrapbook, of five generations of his family. Besides the three Adlais, it covered his great-great grandfather, Jesse Fell, a patron of Abraham Lincoln, and his grandfather, Lewis Stevenson, the Illinois secretary of state, who unsuccessfully sought the Democratic vice presidential nomination in 1928.
Mr. Stevenson, who lived in Chicago, said in a 2017 interview for this obituary that his family political dynasty was almost certainly finished. His son, Adlai 4th, is a businessman and former media reporter in Chicago. A grandson, Adlai 5th, born in 1994, is “a computer whiz,” his grandfather said.
“Neither one has shown much interest in politics,” he added.
Jesus Jiménez contributed reporting.