The 26-year-old Frenchman started 73 seconds behind front runner Haavard Klemetsen following the ski jumping phase of the opening Nordic combined event in Val di Fiemme.
However he executed the perfect race around the cross-country ski course, catching the leading pack with 2.5km to go to put himself into contention for a medal.
And, as the pack dropped from eight to six to four, Lamy Chappuis got stronger and stronger making his break on the last downhill and holding to win after a sprint to the finish with three others.
Lamy Chappuis edged out Mario Stecher by just 0.2 seconds with the Austrian taking silver ahead of Bjoern Kircheisen by 0.1 who claimed bronze ahead of defending champion Eric Frenzel by 0.2.
He claimed his maiden world title in the 10km individual large hill event in Oslo in 2011 a year after winning the normal hill Olympic title and admitted his cross-country tactics had gloriously paid off.
"It feels great, it feels awesome. I couldn't believe it as my jumping was disappointing," said Lamy Chappuis. "But on my skis I felt awesome and I felt everything was right into place.
"[At the finish] I just tried to push hard at the top of the hill and gain a lot of speed. I knew I could stay in front with maybe five metres to go.
"A lot of others couldn't catch me because I gave it everything that I had. I knew it was going to be a hard race with a lot of uphills.
"I started slowly, at a good pace but slowly, but I knew after two or three rounds that I could do it because I had caught up a lot of people."
Lamy Chappuis placed 11th after the ski jumping phase with his leap of 94.5m giving him 107.3 points with Klemetsen leading the way with a distance of 103.5m that earned him 125.6.
Stecher, who has struggled with a knee injury, jumped further than Klemetsen with an effort of 106m but was marked down receiving 121.7 points to start 16 seconds off the Norwegian.
He and Austrian team-mate Christoph Bieler, third after the jumping a second behind Stecher, overtook Klemetsen after just 2.5km with Frenzel, starting sixth 41 seconds behind, in fourth.
Frenzel was less than ten seconds behind by the 3.8km mark and then up to second at the halfway stage as Taihei Kato, fifth after the jumping, came from nowhere to surge ahead.
German Frenzel had the lead by 6.3km with Kato down to sixth while with 2.5km to go as many as eight were bunched together with Kircheisen leading and Lamy Chappuis now with them.
The eight-strong pack was soon narrowed to six and then the remaining four with Lamy Chappuis going early before being reeled in, however his second move proved decisive.
He had the momentum on the home straight with the finishing line approaching and was not to be beaten as he took the win ahead of Stecher, Kircheisen and Frenzel respectively.
Klemetsen was the best of the rest finish fifth 16.2seconds behind Lamy Chappuis in total while Kato was sixth over eight seconds further behind the Norwegian.
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- Mario Stecher