Wirtz left a Chicago law firm to join the Kennedy administration as undersecretary of Labor in 1961. Kennedy promoted him to the top job in 1962 after naming Labor Secretary Arthur J. Goldberg to the Supreme Court.
Wirtz continued in the post after Johnson succeeded Kennedy in 1963 and stayed on until Johnson completed his term in January 1969.
As secretary, Wirtz advocated economic policies to maximize employment, even at the risk of higher inflation. He told reporters at the end of the Johnson presidency that he was most satisfied that the unemployment rate had dropped during his tenure to 3.3% from 5.8%.
In step with the social and economic goals of Johnson's Great Society initiatives, Wirtz's department directed numerous training and education programs aimed at furthering opportunities for workers, particularly the undereducated and underemployed. The Civil Rights Act of 1964 spurred the department to pursue an equal-opportunity agenda, including nondiscriminatory practices by contractors and equal pay for women.
Wirtz spent the final years of his term on the outs with Johnson after articulating his skepticism about the war in Vietnam.
"For two or three years, I was very close to Lyndon Johnson, worked with him on the various programs and worked with him in connection with the 1964 election," Wirtz told National Public Radio in 2008. "And then in about 1966 or 1967, I began to have a feeling that his Vietnam War program did not make sense and should be stopped, and wrote him to that effect. That was the end of the close relationship."
William Willard Wirtz was born March 14, 1912, in DeKalb, Ill. He graduated from Beloit College in Wisconsin in 1933 and earned his law degree from Harvard University in 1937. He taught law at the University of Iowa and Northwestern University.
In Washington during World War II, Wirtz was an assistant general counsel for the Board of Economic Warfare and a counsel to and member of the War Labor Board. He served on the Wage Stabilization Board as chairman in 1946 and executive director in 1951.
As part of a long association with Adlai E. Stevenson, Wirtz was a key advisor during the former Illinois governor's two campaigns for president, in 1952 and 1956, as the Democratic nominee. He practiced law with Stevenson from 1955 until he joined the Kennedy administration.
After leaving the Labor Department he remained in Washington and resumed the practice of law, often serving on boards and pursuing labor-related projects.
Wirtz was married for 66 years to Mary Jane Quisenberry Wirtz, a prominent Washington socialite who was active in political and social organizations. She died in 2002.
He is survived by two sons, Richard and Philip, and two sisters.