The Norwegian had been overtaken at the summit by rival Chiara Hoelzl after the Austrian triumphed in the first event a day earlier as she could only finish fifth.
But the 25-year-old bounced back immediately from that disappointment to top the podium in their second event in Romania, finishing with a commanding total of 259 points.
Lundby jumped 96.5m and 93m to finish ahead of another Austrian in Eva Pinkelnig, whose efforts of 92.5m and 93m saw her rack up a total of 241.1 points to take second spot.
Hoelzl had to settle for third place with a total of 240.8 points, having registered jumps of 93.5m and 93m, as she slipped back down to third in the overall standings.
Lundby now leads the way with 655 points ahead of Pinkelnig, who sits second with 649 points, while Hoelzl has 630 points in a tightly-packed leaderboard.
Stages 19 & 21
Greatest Tennis Players