McIntosh 'in shape of my life' after romping to first Singapore gold
The 18-year-old McIntosh romped home in 3min 56.26sec, nearly two seconds clear of China's Li Bingjie, who was followed by Ledecky in a disappointing bronze (3:58.49).
It was the world record holder's first world title in the event as she pursues five individual golds in Singapore.
On the opening night of the meet, McIntosh and American legend Ledecky, 28, renewed their rivalry from the Paris Olympics last summer.
They will also meet in the 800m freestyle, billed as the most eagerly anticipated race of the championships.
McIntosh took the lead early on and never looked back to demolish her opponents, pulling increasingly clear as the race progressed.
"I'm in the best shape of my life. Now I just have to act on that and put it into all of my races," said McIntosh, who came to Singapore in red-hot form.
The teenager added: "To finally stand on the centre of the podium is promising for the rest of the meet."
McIntosh smashed the 400m freestyle world record at the Canadian trials in June by clocking 3:54.18.
The three-time Olympic gold medallist sliced more than a second off the previous world record of 3:55.38 set by Australian Ariarne Titmus at the 2023 world championships.
Olympic champion Titmus is not in Singapore because she is taking a season off before she dives back in to prepare for the 2028 Los Angeles Games.
McIntosh won gold in the 200m butterfly, 200m medley and 400m medley at the Paris Olympics. She took silver in the 400m free.
"Honestly, I was very relaxed, based off how I've been feeling these last couple of months," McIntosh added.
"Especially in the warm-up today, I felt really strong."
Ledecky, who has a phenomenal nine Olympic golds and 21 world titles to her name, was forced to settle for bronze on Sunday as China's Li surged through for a surprise silver.
"I would have liked to have been a little faster, I was a little faster earlier this year," said Ledecky, one of the best swimmers ever seen.
"There's always that slight bit of wanting to be better than that. I can't complain with a medal."
Ledecky will also race in her signature 1,500m freestyle, in which she is the world record holder and untouchable.
"I'm really excited for the distance races," she said.