Longmuir's delight was matched by Collingwood counterpart Craig McRae's frustration as the Magpies let chances slip in a 7.3 (45) to 5.9 (39) defeat.
In a dour contest frequently marred by teeming rain at Adelaide Oval on Friday, the Magpies led by four points at three-quarter time before Fremantle's Jye Amiss and Matthew Johnson kicked two clutch unanswered goals to snatch a fourth-straight victory.
Star Dockers forward Treacy, held goalless by Billy Frampton, went behind the ball and ran back with the flight to take a huge contested mark as Jamie Elliott flew for a screamer.
"Josh has done that so many times for us - and it's like he's wearing two gloves or put grippo on his hands sometimes," Longmuir said.
"It was pouring with rain, and he goes back to the flight and marks it like it's a dry-weather game. He's been outstanding in that role. It speaks volumes of him to be able to execute when your moments come."
The clutch finish followed skipper Alex Pearce's game-saving smother against Adelaide.
"I feel like over the last maybe 18 months, we've slowly become better at being composed in important moments in the last quarter," Longmuir said.
"I thought we saw some great moments of that."
Fremantle move to 4-1 and second on the ladder, while Collingwood slip to 2-3.
"We've traditionally been really good at managing these moments," McRae said.
"We just didn't manage ours tonight."
McRae couldn't help but lament Dan McStay and Lachie Schultz's missed chances to put the game away early in the final quarter.
"Those guys, in particular, didn't take their chances," he said.
"The game's not decided by those moments, those set shots in particular, I don't want to put that pressure on, but again it does matter at times when you just don't quite execute."
Nick Daicos (31 disposals) returned from a corked calf and played brilliantly through discomfort, while Collingwood's defence was magnificent.
Jordan De Goey copped a knee to the head from Fremantle's Heath Chapman in a second-quarter marking contest, then later smacked the back of his head on the turf when he was pushed over by the defender.
He  failed a head injury assessment and will miss next Thursday's clash with Carlton.
Fremantle ruckman Luke Jackson (38 hitouts, 17 disposals), was excellent, espeically after halftime when he went one-out against Darcy Cameron and Oscar Steene, giving first use to Caleb Serong (24 disposals).
Fellow ruckman Sean Darcy failed a concussion test after he went head-first into Collingwood's Ned Long in the second term and will miss next Sunday's derby against West Coast, likely opening the door for ex-Magpie Mason Cox to debut.
Fremantle defenders Jordan Clark (28 disposals) and Luke Ryan (25 disposals) were excellent along with Alex Pearce (10 intercepts).
Wasteful Collingwood dominated the first term but scored just 1.2 from 18 inside-50s to lead by a point at quarter-time.
Trailing by eight at half-time, Fremantle came out of the break with vastly improved intensity and two goals to Isaiah Dudley helped Fremantle to a nine-point lead.
Jack Crisp put Collingwood back in front at three-quarter time before Fremantle ran riot late.
