Tries for Marcus Lewis, man-of-the-match Jon Lewis and stalwart Dale Keepin, complimented by seven points from the boot of Nick Griffiths, gave Mount their points with Penygraig scoring three penalties by their best player on the night, centre Liam Brabham.
The last quarter-final served up a cracker of entertainment as Cefn Coed defeated Resolven 29-19 at Mountain Ash.
Resolven, unbeaten leaders of Division 5 South-Central were possibly favourites over Cefn Coed from Division 5 South-East, but though centre Rory Healey slotted an early penalty, Cefn captain Ross Sullivan sent coach/fly half Wayne Booth in for a try.
Resolven hit back with another Healey penalty, but Cefn replied as their scrum half Dean Higgs scurried over for Booth to convert.
Resolven dropped too many passes and Cefn's livewire flanker Gwlithyn Thomas scored two tries and wing Jason Wilkes added a fifth try with a Booth conversion.
Healey kicked two more penalties and converted a try by ex-Aberavon wing Daran Griffiths.
In the semis, Cefn Coed play Mountain Ash on April 27 and Maesteg Quins meet Tonyrefail on April 13, both at Llanharan.