The 27 year-old Swiss pushed ahead of his rivals with a bold move before the final climb at the Laura cross-country ski centre and just managed to keep his lead finishing in 1hr 08min 15.4sec ahead of an attacking Marcus Hellner (SWE) who took silver in 1:08:15.8.
Martin Johnsrud Sundby (NOR) and Maxim Vylegzhanin (RUS) engaged in a titanic struggle for bronze with the Norwegian just edging it in a time of 1:08:16.8.
It was a remarkable performance from Cologna, the reigning world champion, who missed part of the season with an ankle injury. It was the second Olympic gold of his career, following his victory in the 15 km freestyle at Vancouver 2010.
A dream come true
“It's great - a dream come true,” said Cologna. “It's very special. I won already here in the World Cup in Sochi, I just wanted to do the same again.
“It's very special for me to win after my injury in November. I didn't expect to be on the podium some months ago. I can't believe I won the first race.”
Not seeded, Cologna had to start just behind the main pack of 20, but he quickly got among the leading bunch and stayed there.
Heavily fancied Petter Northug, a four-times medallist at Vancouver 2010, finally appeared at the front with 5km to go, but he eventually cracked as Cologna, Hellner, Sundby and Vylegzhanin powered away for the last 2km.
Cologna made his break in the final short climb and never looked back, beating Vancouver 2010 winner Hellner by 0.4 seconds.
Despite losing his crown, Hellner was pleased to claim silver: "It feels great," he said. “I had a good feeling throughout the race. Dario was a little bit stronger. I couldn't follow him. I felt a bit stiff in the legs at the end.”
Previously known as the combined 15km + 15km mass start or the 30km pursuit, the skiathlon sees competitors complete a 15km classic style leg immediately followed by 15km of freestyle skating, with a rapid changeover of skis for the different styles.