A passerby spotted the car in Morris, Ill., which is southwest of Joliet, and called 911.
Two police officers pulled the woman from the partially submerged car and did CPR.
"The cop was there in five seconds. We went down there, opened the door, pulled her out," said witness Pat McCabe.
"When firefighters arrived, they did a search. We're confident there was only one person," said Deputy Chief Robert Wills, Morris Fire Dept.
ABC7 Chicago has learned that woman died. Her identity was not being released immediately. It also was not clear why she crashed into the pond.