World record holder McIntosh touched the wall in four minutes 27.71 seconds, far ahead of American silver medallist Katie Grimes and her bronze-winning US teammate Emma Weyant.
McIntosh's triumph comes two days after she took silver behind Australian powerhouse Ariarne Titmus in the 400m freestyle on the meet's opening day.
With her first gold, McIntosh realised the Olympic potential that was promised at the Tokyo Games three years ago, where she narrowly missed out on a podium and came home fourth at the age of just 14.
"Pretty surreal, I was very happy to get the job done tonight as that was a goal of mine to be able to stand on top of the podium and get the gold medal," said McIntosh. "I still feel like I'm just my 10-year-old self.
"I'm just trying to solidify all those dreams I had when I was a kid and I plan on being in the sport for as long as I can do it.
"I still remember growing up and every single day standing up at the beginning of class singing 'Oh Canada', so and now being able to do it at the Olympic Games it's pretty sweet."
Now 17, McIntosh will be counted on to make a considerable contribution to Canada's medal haul in Paris with two more individual events to come: the 200m IM and the 200m butterfly, where she will also be a threat for top spot.
In the first final on an action-packed night at the La Defense pool with five gold up for grabs, McIntosh led from the gun, pulling away from Grimes at every split to come home almost six seconds clear of her nearest challenger.
After the butterfly and backstroke legs, the gold was down to a battle between the Canadian and the American.
McIntosh continued to build her advantage in the breaststroke, and when she made the turn into the freestyle, the fight had already been won as she surged clear leaving Grimes in her wake to hold off Weyant for the silver.
"I couldn't really see anything until the breaststroke second 50 but I kind of looked around to make sure I was in a comfortable place," said McIntosh. "Definitely knew that I had the gold medal."
