The winger is being tipped for international honours this season and it is no surprise given the standard of his finish for the score that ultimately decided this game in the 61st minute.
Of course, it is never that simple for Huddersfield – especially against Saints – as they secured a win over a top-three team for the first time this season.
They have fallen to them late on before and, so, when McGillvary’s fellow winger Aaron Murphy dropped a try-scoring pass with no one near him in the 70th minute, they must have feared the worst.
Similarly, when Murphy –usually one of the most reliable players in Super League – then dropped Travis Burns’s steepling kick in front of his posts, it seemed inevitable that stuttering Saints would recover.
However, there was no such response and, so, the faltering champions have now lost four successive matches.
They still await their opening Super 8s victory to leave them under mounting pressure in the defence of their crown.
Victory, though, is the least Huddersfield deserved as they seek to reach Old Trafford themselves.
In truth, it was an ugly game that saw referee Richard Silverwood award a staggering 26 penalties, a count finished 15-11 to Saints but was far greater in their favour until late on.
The West Yorkshire side had led 14-12 at the break but had spent most of the latter part of the half fiercely defending their own line after conceding seven successive penalties.
They managed to do that until Silverwood finally lost patience, handing out a team warning and then, when Michael Lawrence crept offside between his own posts, handing him a yellow card.
Giants’ rearguard effort finally broke as James Roby – who else? – crawled over from close range with just 64 seconds of the half remaining, Luke Walsh adding his second conversion.
When Alex Walmsley rampaged his way through Jake Connor’s weak tackle attempt straight away in the second half, Saints should have gone ahead but the prop’s pass to the supporting Adam Quinlan was poorly executed and the chance disappeared.
Instead, Danny Brough showed the front-row how it should be done with a perfect ball for Joe Wardle to fire on to, the second-row twisting over in the 44th minute for his captain to make it 20-12.
However, Saints stunned Giants with a back-to-back tries soon after.
Keiron Cunningham’s side, who had been floored by Hull FC here last Friday having lost in Perpignan and the Challenge Cup semi-final, were wretched with their passing among other things, but when Walsh’s latest awful offering bounced to Mark Percival in the 47th minute, there seemed nothing on.
He sent out another pass which was clearly forward as Tom Makinson – the winger rivaling McGillvary for an England call-up and back from a broken leg – stopped low to cross in the corner.
In the next set, Makinson palmed Walsh’s crossfield kick back to Percival and the centre delivered a flick reverse pass for Travis Burns to scramble over, too. Walsh missed both touchline conversion attempts to leave it at 20-20 but when Brough’s attempted kick from a 20m restart went straight into touch, the Australian was handed an easy two points to put them ahead in the 55th minute.
McGillvary struck with his 19th try of the year, though, after a brilliant finish in the corner following a lovely run-around from Jamie Ellis and Jack Hughes.
Murphy added another in the 68th minute after a terrific cut-out pass from the excellent Wardle, but Brough could not convert either so it remained just 28-22 for that tense finish.
Ellis had scored a fine solo try of his own in the 14th minute in response to Kyle Amor’s opener for Saints.
The scrum-half received the ball at first man from a 10m scrum in front of the St Helens posts and put on a big sidestep and dummy to leave the home defence flailing.
Ukuma Ta’ai then bundled over following another fine Brough assist, the captain adding his second conversion and then a penalty, before Saints camped down on their line relentlessly.
Huddersfield’s defence largely stood up to the task, though, and next they host Castleford Tigers –who could draw level with Saints in fourth if they win in Perpignan tomorrow – in a game on September 3 that could go a long way to helping them secure a top-four spot.
St Helens: Quinlan; Makinson, Percival, Jones, Swift; Burns, Walsh; Walmsley, Roby, Amor, Greenwood, McCarthy-Scarsbrook, Thompson. Substitutes: Turner, Masoe, Richards, Savelio.
Huddersfield Giants: Grix; McGillvary, Cudjoe, Connor, Murphy; Brough, Ellis; Crabtree, Robinson, Huby, Wardle, Hughes, Lawrence. Substitutes: Kopczak, Leeming, Smith, Kopczak.
Referee: Richard Silverwood (Mirfield).