By Dave Bettley
Linnets are back in the play-off frame after spectacularly putting to the sword the club they had overcome in away promotion semi-finals in each of the last two seasons.
Billy Paynter's team were full value for the handsome victory, which will have made the rest of Pitching In NPL West sit up and take notice.
Leek had begun the evening above Runcorn in the table but they were overtaken by hosts, who were relentless all over the field and ruthless in attack.
Few sides take Leek apart in this manner and it felt like a night when Runcorn officially announced themselves as promotion contenders again.
Four wins from the last five league matches have lifted them to fourth place, level on points and goal difference with third-placed Clitheroe, who they trail only on goals scored.
Linnets led from the 26th minute, Olly Molloy continuing his hot scoring streak with the first two goals, scoring twice for the second time in four days to make it six in seven since arriving at APEC Taxis Stadium.
Molloy climbed above the Leek defence to head in an inviting Lewis Doyle cross to open the scoring - not easy with one of the Leek centre halves as tall as him, the other two or three inches taller and the goalkeeper taller again.
The striker then consolidated Runcorn's lead two minutes before half-time with another great finish, lashing home a near-post shot from the left of the penalty area.
Intricate work by Eden Gumbs and Will Saxon had opened up Leek's defence, Saxon threading through the scoring pass.
The victory was a nailed-on certainty within five minutes of the restart, Linnets' third coming from Jacques Welsh, who had orchestrated many of Runcorn's best moves.
An exchange of passes with Molloy led to Welsh trying his luck from just outside the area with a low effort which went in off the right post.
Leek had spells of possession at intervals throughout the game but were given hardly a sniff in front of goal.
Each time the visitors got anywhere near, Runcorn busily closed ranks, Peter Wylie invariably on hand to deny them.
Welsh and Doyle were continually pinching Leek pockets, dispossession opponents in midfield to create opportunities at the other end.
The last two Runcorn goals, however, resulted from unforced errors from the away side.
For the first, a mix-up by the visitors at their own throw-in 10 yards short of halfway enabled substitute wideman Adam Moseley to romp clear on the right.
The teenager found centre forward Molloy on the fringe of the 18-yard area and his lay-off allowed Will Saxon to take on the Leek defence wide left.
The winger's 75th-minute cross was met by Doyle, heading home at the near post.
If the first four goals had been well-worked efforts, involving multiple players, the fifth was an explosive solo strike from Saxon.
He intercepted a stray Leek pass out of defence and tore at defenders at pace. His unstoppable shot drew a collective gasp from a disbelieving crowd.
Having flattered to deceive in the first half, the visitors had at least forced Runcorn 'keeper Bayleigh Passant into action the second.
Passant did well to foil Aaron Opuko at close quarters before diving across goal to keep out a last-gasp free-kick, denying Leek what would have been scant consolation as Runcorn continued to threaten.
Enterprising right back Sam Barratt would have scored but for a flying save by Matt Corran, shortly before Leek old boy Saxon brought the scoring to a stunning close.
Next up is a visit on Saturday by struggling 1874 Northwich.
Runcorn Linnets: Bayleigh Passant, Sam Barratt, Eden Gumbs, Anthony Kay, Peter Wylie, Sean O'Mahony (Joe Lynch, 80 mins), Will Saxon (Mwiya Malumo, 89 mins), Jacques Welsh, Olly Molloy, Lewis Doyle, James Hooper (Adam Moseley, 69 mins). Sub not used: Josh Roberts (GK).