Moving where their father’s political career took the family, Adlai III attended five grade and preparatory schools, including public schools in Lake Forest, Ill.; Harrow, in England, when his father worked there with a United Nations group; and Milton Academy, in Massachusetts, from which he graduated in 1948. (Edward M. Kennedy, who was about 16 months younger than Mr. Stevenson, was a Milton schoolmate.)

After earning a bachelor’s degree in government from Harvard in 1952, Mr. Stevenson joined the Marine Corps, attended Officer Candidate School and arrived in Korea with a tank unit just after the truce in 1953. He mustered out as a first lieutenant in 1954. He married Nancy Anderson, who survives him, the next year.

In addition to her and their son Adlai, he is survived by another son, Warwick; two daughters, Lucy and Katherine; his brothers; and at least nine grandchildren.

Mr. Stevenson graduated from Harvard Law School in 1957. After serving as a law clerk for an Illinois Supreme Court justice in 1958, he 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, a nonprofit group, named him the house’s “best legislator.”

Mr. Stevenson went on to be elected Illinois state treasurer, serving from 1967 to 1970. 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, he 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 the United States and East Asia.