A last-gasp try from full back Dean Williams and conversion from outside half Rhys Thomas gave Amman United a deserved win on balance of play, following a committed local derby on a strength-sapping Parc y Werin pitch in very heavy conditions.
Amman had the better of the opening ten minutes, with Rhys Boris' Jones cleaning up in the lineouts early on and Rhys Thomas off-target with two early kickable penalty attempts.
Cwmgors countered to take the lead with virtually their first serious attack; centre Lee Howells' superb midfield break and support from Darren Howells putting wing Carl Carter over for a corner try.
Tom Davies' midfield surge almost paid dividends soon after, but supporting Dean Williams' jinking run came to an end as he ran out of support and was well defended.
Carl Nottingham made some of his trademark short bursts against his former side for Cwmgors, with Andrew Davies also looking lively wide out on occasions.
Good Amman pressure up-front brought reward on the half hour and they forced a penalty, Thomas making no mistakes at his third attempt.
Cwmgors increased their lead near half-time when the referee penalised Amman and yellow-carded Lee Thomas. From the resulting kick, Richard Greig slotted to make it 8-3 at the interval.
Cwmgors, who had brought in props Simon Lillie and Matthew Gillie to solve their front row crisis, spent large portions of the second half on the back foot as Amman's forwards began to get the edge.
Steve Morris, Tom Davies, and Rhys Jones got through some good work; Rhys Thomas' excellent run got them moving and they went through the phases, but Steve Morris, in going for the line, was just held short.
Cwmgors lost Andrew Browsy' Davies to a yellow card early in the second half as Amman turned the screw. Amman almost took advantage, but Stan Manning's powerful run was covered well.
Amman continued to have the upper hand territorially, but were not utilising the ball well. Steve Morris continued to punch holes in the Cwm defence with some surging play.
Rhys Jones, finding himself wide on the flank, almost got through, but he was forced into touch. Cwmgors brought Carwyn Evans, brother of Geraint, on as sub midway through the half.
But his time was short-lived as, following a period of handbags between two committed sides, Carwyn Evans and Amman's Karl Worsfold were singled out for ten minutes' rest, both yellow-carded by Penarth referee David Swain.
Things settled down after the minor dust-up, Amman bringing on Ciaran Looney to freshen the pack, with most of the play still inside Cwm's half. A strength-sapping pitch predictably got heavier as the game wore on.
Cwmgors looked to have weathered the Amman storm, breaking away inside the last five minutes to find themselves pressurising the Amman tryline.
But Wayne Jones and Stuart Herbert helped United counter-attack inside the last minute. Kelvin Green took a crash ball, Stan Manning took it on and, deep into added time, a quick ball spun wide saw full-back Dean Williams come into the line and snipe through for the winning try.
Rhys Thomas, whose kicking was a mixed bag, kept his cool to bang over the winning conversion.
Scrum half Phil Thomas had a solid match behind the Cwmgors pack, but perhaps the visitors deserved the victory on balance of play, to finally stop their recent five match losing streak with a win.
Matchball sponsors were Adrian Brown (Painters and Decorators).
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