LePage, the silver medallist at last year's worlds, began the day in second place but took the lead in the 110 metres hurdles, the first event of the day, and did not budge from that spot to finish with 8,909 points.
"I have done so many decathlons with Damian," LePage said. "We are always supporting each other. It is nice to share this podium with him."
Warner, who won world silver in 2015 and bronze in 2013 and 2019, finished with 8,804 points, while Lindon Victor (30) of Grenada won the bronze on 8,756.
"Am I happy? If you'd have asked me prior to the world championships about taking silver, I'd probably have said no," Warner added. "But if somebody goes out, scores 8,900 points and beats me, I shake their hand.
"Pierce was better and I'm very happy for him. But I'm very proud of myself, too. I wanted to get the gold, but it makes it a little bit better to have another Canadian winning it.
"Pierce is a great guy and it will be great to stand on the podium with him and our 'honorary Canadian, Lindon Victor."
World record holder Kevin Mayer (31) of France withdrew after the first two events with an Achilles injury suffered two week ago.
LePage served notice when he started Saturday's events with a personal-best 13.77 seconds in the hurdles, and then built on his lead with second-best results in the discus (50.98 metres) and pole vault (season's best 5.20).
"I had a couple of close calls. First, when I landed in the pit during long jump, I twisted my shoulder. It was rough. Then I got cramp at 1.99 metres during the high jump. This morning I pulled my hamstring before the hurdles," LePage said.
"But it is decathlon, we're used to a lot of injuries, you get through it. I feel like I've not been sleeping for the past three days and I will celebrate this gold with a good sleep."
Warner made up ground with his sixth-best finish in javelin - one spot ahead of his rival - but LePage had a 184-point lead heading into the final event, the 1,500m. LePage crossed 12 seconds behind Warner but the gold was already well in hand.
Warner led through four events at last year's worlds in Eugene before pulling up with a hamstring injury in the 400m.
"This silver motivates me a lot. Decathlon has its ups and downs and it feels great to finish with more than 8,800 points," said Warner. "This result puts me into a good position going into next year."
