Lajunen used his 15 second advantage gained from winning Thursday's ski jumping portion to cruise to victory in the 7.5km sprint.
The Finn came home in 16 minutes, 40.1 seconds, finishing nine seconds ahead of Germany's Ronny Ackermann, who claimed silver.
But Felix Gottwald's performance on Friday was one of the highlights of the event, climbing eight places to grab third.
"I wasn't really thinking about a medal. I was concentrating on a good race"
Finland's Jaakko Tallus took fourth, followed by American Todd Lodwick.
But it was a historic win for Lajunen, who has become the most decorated Olympic nordic combined athlete of all time.
The 22-year-old had already won gold medals in the two previous Nordic combined events at Salt Lake City as well as an individual silver and team silver at Nagano in 1998.
His medal tally eclipsed Ulrich Wehling of East Germany who amassed three gold medals in Nordic combined.
But the Finn said that medals were not his first priority.
"I wasn't really thinking about a medal. I was concentrating on a good race," he said modestly.
Lajunen, who won the men's 15km and team gold medals earlier in the Games, had done most of the work on Thursday.
He landed a stylish 126.5 metre jump for 123.8 points to carry over his crucial lead over runner-up Ackermann, who was second after ski jumping.