McIntosh finished in a time of 3:56.08 at the Toronto Pan Am Sports Centre to beat Titmus's time of 3:56.40.
"I'm not a crier but this world record, I didn't think this was possible tonight," McIntosh told CBC Sports.
"It's absolutely incredible. I'm not an emotional person. But I was hit with so much emotion. Pure euphoria right now.
"Over the past few years, I've put my life into this. To be the best I can be. To achieve something like this, it was very unexpected. It was never in my dreams to do this tonight or even a few years ago. This just blows my mind."
McIntosh's head coach Brent Arckey said the Toronto native was "one of the best racers I've ever seen".
"I'm not really sure what to say right now. I'm trying to hold it all in. But celebrating her. This is a special thing," he told CBC Sports.
Earlier this month, McIntosh ended American Katie Ledecky's (26) nine-year home pool winning streak when she touched first in the 200m freestyle at a Pro Series swim meet in Fort Lauderdale, Florida.
