Lower-ranked visitors Llanfairpwll were five minutes away from causing a Welsh Cup sensation at Eirias Park.
The Anglesey side from North Wales Coast West Division One led 1-0 until the 85th minute, when tier 4 Mochdre converted a penalty which forced a shoot-out.
And it was the North Wales Coast East Premier Division hosts, third at that level last season, who prevailed on spot-kicks.
Sports made a few changes from the previous game, a 2-0 midweek league triumph over Penmaenmawr Phoenix, with Rees Brown coming in for his first start of the season and Owen Williams returning at left back with Alex Bowell switching to his more comfortable right back slot.
Bobby Tudor was back in the line-up in midfield, and subs on Wednesday, Ste Falvey and Jordan Taylor, started on Saturday.
It was a tense affair with a blustery wind affecting things for both sides.
Llanfair deservedly took the lead on 36 minutes when they beat the offside trap, Jamie Patterson finding the net.
Mochdre left it late to draw level when Charlie Lloyd was fouled just as he was about to shoot. Sub Josh Grimshaw scored the resulting penalty.
The game went straight to pens, and Mochdre keeper Nic Bould saved the day again, keeping out two of the visiting side’s penalties.
Grimshaw, Owen Woodfield, Rees Brown and Gaz Jones all bagged pens for Mochdre, to send them through 4-3.
But yet again, a tier 5 side from Anglesey had done their league and the island proud.
Nic Bould
Alex Bowell
Owen Williams
Rees Brown
Gareth Jones (c)
Jack Jeffers
Ste Falvey
Robert Tudor
Jordan Taylor
Scott Brown
Christopher Lloyd
SUBS USED
Joshua Grinshaw
Micheal Corrales
Owen Woodfield
Charles Lloyd
Hayden Shilton
Thomas Wilkes
Aled Davies
James Couchman
Ieuan Grenier
Dylan Williams (c)
Jamie Patterson
Adam Thomas
Marquis Holland
Rees Roberts
Carwyn Roberts
Cian Owen
SUBS USED
Danny Connolly
Dei Thomas
Thomas Mummery
Brady White
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