Sweden's William Poromaa battled all the way but was passed coming into the final turn and had to settle for silver, while Klaebo's compatriot Simen Hegstad Krueger took bronze in a tense race resembling a Hollywood thriller over almost two hours.
"It's been crazy," Klaebo said at the finish line as he struggled to hold back tears of joy. "I feel like I've been working so hard to just be here and try to be in my best shape and managing that, and winning six out of six here. It's just crazy. I don't have any words."
Fans began queuing in the early morning as a crowd estimated by media at over 100,000 filled the stands and lined the course, creating a carnival atmosphere for one of the classic races in a sport that is followed fanatically in Norway.
While the bright sunshine warmed the crowd it made conditions difficult for racers, who had to deal with wet, heavy snow that was quickly carved up and demanded every ounce of their power on the punishing uphill stretches.
After a slow start and nip-and-tuck first 25km, it took until the 35km mark before a five-man breakaway featuring Norwegians Klaebo, Krueger, Harald Oestberg Amundsen and Martin Lowestroem Nyenget plus Swede Poromaa took control of the race, leaving the field in their wake.
Nyenget looked strong and attempted to break with three kms to go but was reined in and, having suffered a fall in the skiathlon race that cost him a medal a week ago, took another late tumble in the loose snow to ruin his chances.
The tremendous tactical battle evolved further when Klaebo switched lanes to make a burst into the final climb, but Poromaa answered and made an surge of his own as their pair dropped Krueger.
Unfazed, Klaebo let the Swede briefly take the lead before blasting by his rival on the way into the final bend and thrusting through the final straight to claim his historic sixth gold medal as the stands exploded in jubilation.
"I feel like I got helped by the hundreds of thousands of people out there. It was crazy, I feel like I say this, every single race that this is, that this has been the best day so far, but today it's ... it's just crazy," Klaebo said.
