Josh Earl put the Lilywhites ahead close to half-time before Anthony Plant pulled one back for the hosts just after the hour, but it was Garstang’s finish with just nine minutes remaining that proved to be the deciding factor in this well-contested affair.
Both sides started brightly, with PNE going close through a Precioux Ngongo header and Dylan Davidson’s low drive early on. But Callum Roberts was kept busy in the away goal too as he was forced off his line to sweep up a Wigan counter attack before he superbly denied Tom Powell’s effort one-on-one.
Powell continued to threaten for the hosts as he saw another of his efforts denied – this time via Tom Stead’s superb sliding block – before Roberts thwarted the Wigan man again by holding onto his dangerous low cross midway through the half.
Stead was then forced off with injury but was replaced by Mike Howard in a positive and attacking change.
And after some key pieces of defending from Kamol Campbell just after the half hour mark, Howard’s introduction almost helped North End to get the opening goal. His cross just evaded Davidson at the far post after impressive link up play with Earl on the left as it was cleared for a corner.
But PNE took the lead from Melle Meulensteen’s resulting corner, his delivery – helped by the goalkeeper’s attempted clearance – falling to Josh Earl to control and fire into the far right corner.
That boosted the visitors and they may have doubled their lead before the break, as Howard curled one wide from the left before Oscar O’Neil placed a shot just over the bar from Ki Smart’s lay-off on the stroke of half-time.
North End continued to threaten after the interval as Davidson’s attempted lob was held and some terrific Earl crosses went untouched inside the area.
But Wigan got the next goal in the 62nd minute, as Matthew Taylor’s left-wing delivery reached Plant who stabbed in at the far post.
Campbell was injured in the build-up to that goal and replaced by Garstang in another attack-minded substitution, but Wigan took the momentum from their equaliser as Roberts was put under pressure in the PNE goal.
But he showed his quality to deny Mwiya Malumo from just yards out before Davidson’s cross just evaded Garstang at the other end.
The home team went close with a volley going marginally over in the 78th minute before Davidson and Garstang combined again three minutes later – this time resulting in the vital goal.
Davidson hit another dangerous, low cross from the left and the quality of his delivery gave the second-half substitute a simple task; tucking in with his left-foot from close-range.
That duo combined again as Garstang pressed the Wigan defence and set up his Northern Irish teammate, who saw his shot saved.
Fellow late substitute Tom Barry also went close in the latter stages, shooting wide from just outside the box and then heading wide from a Meulensteen free-kick.
Wigan pushed for an equaliser and a makeshift-looking PNE backline coped well, although Roberts made another great save from a shot curling towards the bottom corner late in stoppage time.
North End held on for an impressive away win, and assistant manager Andy Fensome was happy with what he saw from the team.
“I thought we started really well in first five or ten minutes,” he said. “We had two or three chances and half chances in the box.
“We’ve looked at the stats and it was 50-50 possession in the first half. I thought in the second half we got into shape better out of possession and then at times if we had been a little more relaxed and composed on the ball, we could have broken them down more easily than we did.
“We scored a great goal with a few passes together, a cross into the box and a good finish.”
Fensome agreed that PNE were deserving of their win but admitted that goalkeeper Callum Roberts had made some important saves for the Lilywhites.
He added: “They had a period of probably 15 minutes where they opened us up a few times and Callum Roberts made some excellent saves.
“After that, I felt the game was fairly even. It was fairly open, I said that to the boys at half-time.
“But in the second-half, I thought we were more compact and with just a bit more quality in possession, we could have won the game more easily.”
And after defenders Tom Stead and Kamol Campbell were injured during the game, Fensome was happy with how PNE’s makeshift backline coped with pressure from Wigan’s attack.
He said: “What we try and instil into the boys is that it doesn’t matter where you’re picked to play.
“If you can play you will adapt – it’s not something that we haven’t done in training during the week at different stages.
“But to finish with a back four of Leon Creech, Ngongo who is an under 16, Earl who went from left-wing to left-back to centre-back and Howard; I think they actually coped quite well second-half.”
PNE line-up: Roberts, Creech, Stead (Howard, 25), Ngongo, Campbell (Garstang, 64), Smart, O’Neil (Barry, 80), Wood, Meulensteen, Earl, Davidson. Subs not used: Lenton, Kwateng.