After breaking into the truck, two offenders moved mail and packages into a black GMC Suburban parked in the 5900-block of W. Fulton Blvd.
The off-duty officer followed the offenders as they fled and dialed 911, providing the dispatcher with information.
When police officers began monitoring the vehicle, one offender attempted to throw mail onto the street. The offender fled from the vehicle.
Chicago police arrested one of the robbers, Darrick Smith, 43. He was charged with one count of burglary.
A large chunk of concrete was found next to the US Postal Servicel truck, with a broken window on the driver's side door.
"The valiant actions of all officers involved in this case exemplify the dedication and commitment officers display on a daily basis," said Austin District (15th) Commander Barbara West.
Chicago police say that packages containing expensive items and personal information are often targeted by thieves during the holidays. Residents are asked to stay alert and report any suspicious activity.