The first Premiership play-off game since 2002 - when Caerphilly defeated Aberavon over two legs to retain top-flight status - got off to a sensational start. Bonymaen captain and hooker Darren Poland was sent off in the third minute for head-butting Maesteg lock Paul Jones in front of the grandstand, but the wind-assisted Parc Mawr side then dug deep, and had much the better of the opening half.
They took a 22nd-minute lead when fly-half Gareth James placed a penalty, and five minutes later Maesteg lock Gavin Ronan received a yellow card off Llanharan referee Phil Fear. Bonymaen increased the lead via a second James penalty in the 35th minute, but a bout of fisticuffs then led to Bonymaen wing James Davies along with Maesteg captain and flanker Ian Boobyer being binned.
Maesteg, who trailed 6-0 at the break, hit the front four minutes into the second half.
Drives close to the try-line from back row men Marcus Sainsbury and Nathan Strong were held up, but scrum-half Kevin Ellis dived over for the try.
Fly-half James Dixon, playing a final game before joining the Dragons on a development contract, added the conversion, but in the 48th minute Bonymaen's once-capped Wales full-back Darril Williams and Maesteg wing Neil Dixon were both sent off, following another fight.
A 63rd-minute penalty from Maesteg fly-half Dixon then extended the lead to 10-6, and the Old Parish closed things out when Dixon landed three late penalties.
Tries & Pens: G James (2)
Tries - K Ellis
Con - J Dixon
Pens - J Dixon (4)