Sweden finished second while a team of Russians were third.
The 37-year-old Bjoergen now has 13 medals, which ties male biathlete Ole Einar Bjoerndalen, also from Norway. The 44-year-old Bjoerndalen participated in six Olympics but did not make this year's team.
Bjoergen was 3.4 seconds behind with the Norwegians in third place entering the final leg of the race. But she quickly took the lead and never let it go, holding off Stina Nilsson for the gold.
She has two events remaining in Pyeongchang.
It was Bjoergen's third medal at this year's Olympics, but her first gold. She also has a silver and bronze medal in individual events.
The Norwegian great now has seven career gold medals, which leaves her only one behind Bjoerndalen and Bjoern Daehlie for most in Winter Olympic history.
Bjoergen can become the most decorated Winter Olympian ever with a medal in either one of the final two women's events — the women's team sprint relay or the 50-kilometre mass start.
Bjoergen, at 37 years, 333 days old, became the second-oldest Olympic champion in the event behind only Raisa Smetanina, who was 39 years, 354 days old when she claimed gold in 1992.