Quite a surprise at the Flyover Ground as Junction lost for the first time in 14 league matches.
Llandyrnog, by contrast, had won just one of their previous nine, so the form book was turned upside down on Saturday.
Junction started quickly and took the lead after two minutes when Robert Gore scored with a audacious overhead kick which left the keeper rooted to the spot.
This seemed to wake Llandyrnog up and they reacted in the right way, getting a 10th minute equaliser through Jamie Jones.
United continued to apply pressure and were rewarded with the lead through Osian Audley. The winger spotted the keeper off his line and hit a swerving strike over his head.
The Railwaymen were back on level terms after 27 minutes when Matty Jones had a free header from a corner.
It had been a back and forth battle for the first 30 minutes but Llandyrnog had a spell of dominance before half time and took control of the game.
Audley grabbed his second after some good work from David Lawson Cooke, who made it 4-2 with a calm finish before the break.
At the start of the second period Junction came out looking like a new side and went on the offensive right away, but the Llandyrnog defence looked solid throughout, repelling wave after wave of attacks.
Shot stopper Jake Jones produced some great saves.
Llandudno Junction pulled a goal back on the 60-minute mark as Harry Parkes-Smith fired through a crowd of bodies to make it 4-3.
The hosts pushed hard for an equaliser, but it failed to come.
Goals from Lewis Brownson and William Hughes secured the points which edged Bro Cernyw further away from the bottom two.
Brownson broke the deadlock on 40 minutes and although Gethin Roberts levelled soon after, Hughes’ strike after 76 was the winner.
Llansannan had Leon Gierke red-carded after the final whistle.
Sports are up to fourth after they avenged a 5-3 home defeat at the hands of Cerrig inflicted three weeks earlier.
Antonio Jacovelli and Kris Owen gave the visitors a two-goal half time lead, then substitute Christian Kelly struck twice after the break.
Ryan Gordon (40 minutes) and Andrew Merner (63) sealed the win for third-place Rhuddlan at the bottom side.
Glan had Joel Morgan Jones sent off for two yellow cards seven minutes from time.
More points dropped in the title race for Llannefydd, but a great draw on the road for Llanfairfechan.
Andrew Thomas and Ciran Jones found the net for the hosts, Harry Croston and Cory Young (penalty) rescuing a share of the spoils for the visitors.
Mochdre led through Tom Paddock’s 36th minute net-finder, but Daniel Williams pounced on 77 to grab Phoenix a draw.
Loui O’Carroll’s 85th minute decider bagged another three points for an in-form Bow, who made in six straight wins and nine games without defeat.
Caer Clwyd put up a great battle with a number of fine stops from John Wood keeping them in the contest.
On his birthday weekend, Colin Sargerson notched a crucial fifth goal in the 84th minute which clinched maximum points for the Dragons.
Tom Jones (2) and Macca Lewis were among the other Dragons marksmen, Jack Such, Solomon Dickens and Josh Robinson bringing Penrhyn Bay back to 3-3 at one stage.
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