In 1933, Mitchell was hired as the Director of Research for the Illinois Tax Commission. In 1940, he became Assistant to the Director of Revenue for the State of Illinois.[1]
In 1943, Mitchell joined the staff of the Federal Reserve Bank of Chicago, but was also as a consultant for the Bureau of Internal Revenue. During this time, Mitchell simultaneously served as Director of Finance for the State of Illinois under Governor Adlai Stevenson. He was later appointed as the head of research for the Chicago Fed.[1]
In 1961, President John F. Kennedy appointed Mitchell as a Governor for the Federal Reserve Board.[2]
In 1973, President Richard Nixon appointed Mitchell as the Vice Chair of the Federal Reserve.[2] He served under Fed Chairman Arthur F. Burns. Shortly thereafter, he was thrust into a difficult position of resisting a guarantee of the municipal bonds of New York City, echoing the stance favored by President Gerald Ford. Mitchell stated that a guarantee would be akin to a bailout and would subject the Fed to all manner of political pressures.[2] Ultimately, the federal government helped New York City avoid bankruptcy with a loan and debt restructuring by the Municipal Assistance Corporation in 1975.
Mitchell was a member of the American Economic Association, American Finance Association, and National Committee on Government Finance, Brookings Institution.[1]