The 17-year-old produced a brilliant late surge to win in a time of 2mins 06.56sec, the third-fastest time in history.
Kate Douglass of the USA finished second to take silver while Australian Kaylee McKeown took the bronze after American Alex Walsh, who had finished third, was disqualified.
McIntosh had already claimed gold in the 400m individual medley and the 200m butterfly, as well as a silver in the 400m freestyle.
McKeown, who was also looking for a third gold in Paris, was viewed by many as her main threat but Douglass was in the lead after the opening butterfly leg.
McIntosh led from McKeown after the backstroke before Walsh grabbed the advantage at the last turn, following the breaststroke leg.
But McIntosh once again produced the killer final lap to grab a thrilling victory.
The silver for Douglass follows her gold in the 200m breaststroke.
