Two missed penalties by Boyd from easy angles gave Rangataua hope they could still win, trailing 18-11 but a yellow card for punching proved costly as Greerton's impressive pack took control of the final stages.
The match-winning try came from a classic lineout move with No8 Joe Key breaking around the side and down the touchline to crash over in the corner. First-five Josh Evemy slotted the conversion expertly from touch to finish off the men in black.
Rangataua started well enough and gave plenty of ball to their biggest threat in left winger Siaosi Iongi, who has been in impressive try-scoring form in 2014. The former Tongan sevens rep scored in the first half to help his team to 11-3 in front, but he was well tackled in the second spell.
Rangataua were missing influential captain Matt Clutterbuck and Matua Parkinson but Greerton too were missing big guns. They showed they are building good depth with players stepping in to replace Bay of Plenty reps Tino Nemani and Joe Tupe, plus first-five Tim O'Malley.
Greerton Marist 25 (Andrew Boyd, Tyrone Lefau, Joe Key tries; Boyd con, 2 pens' Josh Evemy con) Rangataua 11 (Siaosi Iongi try; Daiyu Ishimori 2 pens)