Emanuel "jumped up from his seat, and helped him lie down on the floor," passenger Aron Weisner told the Chicago Sun-Times in an account published Thursday.
Weisner said he was sitting next to Emanuel in coach on the United Airlines jetliner when the man across the aisle began to hyperventilate. The mayor notified flight attendants of the situation and offered assistance to the stricken passenger.
David Brown told the Sun-Times Emanuel raced to the front of the plane, where he grabbed a blanket "from an overhead compartment in first class and put it under the guy's head."
The flight attendants asked for medical help and two people responded, witnesses said.
"He [Emanuel] was the only passenger actively engaged, other than the doctors," Brown said.
The man was able to walk off the plane when it landed in Washington, said Brown.
Emanuel was on an East Coast swing to raise money for his own political war chest and to stump for fellow Democrat Cory Booker, the Newark, N.J., mayor running for the U.S. Senate.