The result turns Group E into a three horse race, with Brazil facing Hungary on Thursday and Nigeria against North Korea.
Mervo opened the scoring with a near post header in the 17th minute, after the North Korean defenders seemed to switch off. Choe Ju Song got a well worked equaliser in the 32 minute but that was as close as the Asian side got, as Zsolt Kalmar restored the Hungarian advantage a minute later.
The powerful Mervo added his second just after halftime, followed ten minutes later by compatriot David Forgács. North Korea never stopped trying but conceded a fifth in the 82nd minute, with Mervo scoring from an impossibly acute angle after a improbable dribble along the by-line.
Hungary 5 (B Mervó 17, 49, 82, Z Kalmár 33, D Forgács 60)
North Korea 1 (C Ju Song 32)
Halftime: 2-1