The Dockers struggled for fluency in their 36-point loss to Gold Coast in the rain last round, and a pre-match downpour on Sunday made for another slippery affair.
The Magpies were dealt a blow midway through the second quarter when ruckman Mason Cox was subbed off with a dislocated finger.
But his replacement Oliver Henry became an unlikely hero, with the 19-year-old booting four second-half goals to help propel Collingwood to the 12.8 (80) to 6.8 (44) victory.
When Sunday's game was at its wettest - in the opening term - Fremantle dominated a host of key stats and scores were even.
Longmuir feels the problem lies in his team's inability to adapt to changing conditions.
"As the game went on we were just getting caught in-between with the ball," Longmuir said.
"Whether it's the actual conditions or we think it's wetter than it is or drier than it is, I think that second quarter we just fuelled their forward game and fuelled their field position.
"We turned the ball over in really poor areas going for stuff that in the conditions wasn't on, and then as it dried up we kept playing wet weather footy when they were playing dry weather footy.
"I think it is a mental thing.
"Players read too much into the conditions and it affects their games too much, especially with ball in hand."
Will Hoskin-Elliott, Brody Mihocek and emerging star Jack Ginnivan shared the load up forward for Collingwood with two goals apiece.
But it was Henry who shone brightest in attack, with the forward booting three goals in the final quarter alone to kill off the Dockers.
It came just a week after he booted a wayward 0.3 in a 48-point loss to the Western Bulldogs.
"It's a great lesson for resilience," Collingwood coach Craig McRae said.
"Last week he kicked (0.3) and I spoke to him and said you have to be more present and make the most of the opportunities.
"One of the hardest things in the game is getting the ball in your hands. He did that last week; today he looked like a player who had more composure."
McCrae said Cox's dislocated finger had protruded through the skin, with the American set to be sent for scans to determine the extent of the damage.
Ginnivan earned the ire of Fremantle fans in the final term when he outmarked Brennan Cox and celebrated in front of the Dockers' faithful while he strolled in to kick the easiest of goals.
The win improved Collingwood's record to 5-5 ahead of next Sunday's MCG blockbuster against arch rivals Carlton.
Fremantle started the season with a stunning 7-1 record, but back-to-back losses in the wet have raised serious questions about whether their game plan can hold up in slippery conditions.
Will Brodie (36 disposals, 11 clearances) and David Mundy (35 possessions) led the way for Fremantle.
Collingwood lost the possession count by 49 and contested possessions by 23.
But their efficiency with the ball meant they won the inside 50m count 60-46, allowing their forwards to kick a winning score.
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