With today being Rahm Emanuel’s first day on the job as mayor of Chicago, news media have labelled him the first Jewish mayor of Chicago. While the distinction of being first elected Jewish mayor of Chicago may still belong to him, the first acting Jewish mayor of Chicago was Samuel Adams Ettelson:
Jewish Attorney Chicago’s Acting Mayor (July 29, 1928) Chicago. III - (Jewish Daily Bulletin)
Corporation Counsel Samuel A. Ettelson has been named Acting Mayor of Chicago in the absence of Mayor William Hale Thompson, who is suffering from ill health. Mr. Ettelson is to be in power until after Labor Day, at which time it is expected that Mayor Thompson will be able to resume his duties.
This is the first time in the history of the city that a Jew has been at its head.
As the city’s corporation counsel, Ettelson codified the Chicago Municipal code of 1922, which can be viewed in its entirety on Google Books. Others have pointed out the fact that Illinois in the 1930s had a Jewish governor, Henry Horner, who is also documented in our archive.
For an interesting analysis of Chicago Jewish politics (save for the fact about Ettelson), see Edward McClellan’s blog, "Ward Room" and this September 2010 profile of Rahm Emanuel in the Chicago Jewish News.