Harvey Knapp, who is from England, became Highwood's 1st Ward alderman in October when the council unanimously approved him to replace Alderman Quintin Sepulveda, who resigned, the Chicago Tribune reported Thursday.
However, Knapp, who has been living in the United States for 23 years but is not a citizen, discovered while going through paperwork with Highwood Mayor Charlie Pecaro about a potential run for a full term in April that candidates are required to read a loyalty oath swearing they are U.S. citizens.
The men soon discovered Illinois Election Code states all municipal officials must be registered voters, which requires U.S. citizenship.
The council is expected to accept the resignation during its Thursday meeting.
City attorneys said votes Knapp participated in during his time on the council will still be considered valid.