Whangarei started with an early try but then made a series of errors including a poor pass that led to a Matt Tauatagaloa intercept try - which encouraged the home side and kept them in the game.
The game continued to see-saw with Whangarei just ahead at the break, 22-17, before the Mangonui outside backs were given space in the second spell to forge ahead.
Mangonui manager Adrian Jones said his players were kicking themselves after the match.
"We're happy with how we played in general, particularly with the way Mangonui have travelled over the last few years, but we had them right there for the taking but we let it slip so we were pretty disappointed."
He said a couple of ill-directed kicks late in the game had given Whangarei the opportunity to get back into the lead and they had executed a good comeback.
The early match at Arnold Rae Park was a more one-sided encounter which saw Northern Wairoa-Rodney defeat Bay of Islands 38-0.
The next round of games is at Kaikohe on June 7, followed by the final round at Whangarei on June 28.
Whangarei 36 (Ume Fafita, Dan McCully, Waipu Stone, Brook Gilmore (3) tries; Tyler Lansdowne 3 con) Mangonui 32 (Matt Tauatagaloa, Jone Macalai Tori, Toby Lynn, Filipo Nakosi (2) tries; Hohua Clark pen, 2 con).
