Court documents show that a Chicago man entered the state senator's Oak Park office on North Avenue on Friday.
The man, identified as Scott Loeffer, took out a knife and pointed it toward a female victim before taking her iPhone, according to the documents.
Loeffer has been charged with armed robbery.
Harmon, a Democrat who represents Illinois' 39th District, issued a statement, saying the victim, a staffer, was not injured. He added that the incident does not appear to have been politically motivated.
Further information was not immediately available.
Harmon's full statement:
"On Friday, a man entered the district office and confronted an employee with a knife. Thank God she is physically unharmed. The suspect is in custody and charged with armed robbery.
This incident does not appear to have been politically motivated.
I want to thank the Oak Park Police Department for a quick response, and I want to thank my staff for their bravery and unwavering commitment to public service. Their safety is my top priority."