The short answer is during the 1950-1951 school year and Assistant Professor Robert O. Gibbon.
The question is  “When did the department begin and who was the first chair?”

This blog will be used to record our research on departmental histories as they are received. A backlog currently exists in this type of research and I will catch it up in a post as soon as possible. For now, here are some added details in the development of PoliSci at IWU:
The 1921 school year shows the earliest mention of a political science branch within a division of the Department of Economics and Sociology. It was headed by Professor Carl W. Strow.

The 1922-23 school year lists political science as part of the Department of History and Political Science, marking the first time it was in the title of a department. This department was headed by Professor William Wallis and this structure/alignment and chair remained the same until 1950. At that point Wallis remained History’s Chair and Gibbon (who had been on the faculty in the combined department since 1947) became chair of the new, separate department.

Other chairs:
1955-1958 Robert O. Byrd
1958-1963 A. Glenn Mower, Jr.
1963-1967 Bunyan H. Andrew (dept shows combined with History again during this time)
1967-1972 Donald P. Brown (who had been teaching in Hist/PoliSci during the recombined period)
1972 John Wenum begins