Jake Hyde's second in two games had been cancelled out by Enzio Boldewijn shortly after County had a penalty saved.
As far as tests of Town's play-off credentials go, a Tuesday night at Meadow Lane takes some beating.
Town did not look overawed in the slightest in the, by National League standards, grand surroundings, with Jamie Allen, the only change from Wealdstone, latching onto Jake Hyde's flick on and bringing a decent save from Sam Slocombe within the opening two minutes.
After a brilliant jinking run, Jack Earing then forced Slocombe into another save a few minutes later, with Town dominant and repeatedly making inroads down the left with Jack Senior.
Time and again, County's attempts to play through midfield were thwarted by Town's pressing as they swarmed around the hosts.
It was brave from Halifax, who hadn't come to sit back and soak up pressure, but instead to impose themselves on the game. Mission accomplished.
Jordan Barnett fired narrowly wide from range for the hosts shortly before Hyde and Martin Woods had efforts off target for Town.
Kieran Green was once again relishing his role in the midfield engine room, with Senior the visitors' best attacking outlet - giving 21-year-old County right-back Kenton Richardson a fairly torrid debut - and again reaching the byline before King met his cutback and saw his shot blocked.
County headed their first corner narrowly wide, but that was against the run of play.
And Hyde capped a dominant opening 30 minutes from Halifax when he converted Earing's cross from close range after the midfielder had burst into the box.
It was the least The Shaymen merited for a blistering first half-an-hour.
Sam Johnson, making his 200th league appearance for The Shaymen, was finally tested with Ruben Rodrigues' fairly tame shot from 20 yards, but County looked a frustrated, disjointed outfit.
Halifax looked like they could run all night.
Johnson then kept out Boldewijn's stinging drive from an angle but County desperately needed a half-time rollocking.
The difference in the sides was emphasised shortly before the break when Town's work-rate and desire saw Green and then Earing hassle and harry County players into passing straight out of play.
The interval was a relief for the home side, but stopped Halifax in full flow.
After the break, it was more of an even game. The home side still struggled to create, but were more aggressive and pushed Halifax back.
The excellent Senior was taken off, presumably injured, on the hour mark, and replaced by Billy Chadwick, with the reshuffle seeing King switch to the left and Allen back to right wing-back.
But Halifax remained solid and pretty impenetrable, with their back three pillars of a resolute rearguard, while Hyde again caught the eye by providing a crucial link from front to back.
It was a far cagier second-half, which suited Town just fine.
The only negative from the visitors was their passing had become more ragged and there was less composure about their play, which, combined with a better half from County, made the lead look slender.
The advantage was preserved when Johnson kept out substitute Elisha Sam's penalty low to his right after Allen's foul in the box, although Town will argue justice was done and that it was a soft decision.
But it was only a temporary reprieve as, a couple of minutes later, Boldewijn drove an unstoppable, powerful low shot beyond Johnson from the edge of the box, and 75 minutes of resilience was wiped out.
But moments after coming off the bench, Campbell struck a piledriver across goal into the far corner to win it in style.
Play-off contenders? You'd better believe it.
Notts County: Slocombe, Richardson, Lacey, Rawlinson, Barnett, Reeves, Doyle, Rodrigues, Boldewijn, Knowles (Sam 59), Wootton. Subs not used: Ellis, Kelly-Evans, O'Brien, Miller.
Scorer: Boldewijn (75)
Shots on target: 4
Shots off target: 6
Halifax: Johnson, Byrne, Maher, Bradbury, King, Woods, Green, Earing (Tear 89), Senior (Chadwick 59), Hyde, Allen (Campbell 83). Subs not used: Renshaw, Benn.
Scorers: Hyde (28), Campbell (84)
Shots on target: 8
Shots off target: 5
Referee: Adam Herczeg
Town man of the match: So many to choose from, singling one out seems unfair, but Kieran Green gets the nod for another performance full of energy and industry. Epitomised Town's spirit.