Brett Kahn was sentenced to two years probation after the guilty plea.
The Chicago Police Board voted in April of last year to fire Kahn for a separate excessive force incident that occurred a month before the baton attack.
Kahn surrendered to sheriff's deputies four years ago after
being charged with felony aggravated battery and official misconduct stemming from a melee captured on camera in the summer of 2014 in Chicago's North Lawndale neighborhood.
Prosecutors said Kahn
hit 32-year-old Jeremiah Smith without provocation while he was on patrol responding to a block party celebrating record deals signed by two neighborhood teens with a major Atlanta label.