The Dockers kicked into gear with nine goals to one after a halftime "tune-up" from coach Longmuir, coasting to a 16.12 (108) to 7.5 (47) victory in front of 10,028 fans at Lyndoch on Sunday.
Andrew Brayshaw (30 disposals) starred alongside Caleb Serong (29) and Jaeger O'Meara (21) in the midfield, while creative recruit Shai Bolton was dangerous with 2.3 from 19 disposals against his former side.
But Treacy was the headline act, kicking three goals in the first 12 minutes of the match on his way to a personal-best half-dozen.
Longmuir hailed Treacy's work ethic and selflessness, tipping there's plenty more to come from the 22-year-old key forward.
"He's a phenomenal athlete, a really smart footballer and a real competitor," Longmuir said.
"I don't judge his game on how many goals he kicks, but if he keeps providing options and working as hard as he did today, he's just really hard to defend."
Fremantle stars Sean Darcy and Hayden Young got through their first games back from injuries, with Young kicking two goals from 18 disposals.
Longmuir dismissed concerns over All-Australian defender Luke Ryan, who had a sore left shoulder and was subbed out at three-quarter time.
The Dockers trailed by 11 points midway through the second quarter, but kicked 12 of the next 13 goals to run away with a third straight win.
"(I gave them) a little bit of a tune-up at halftime about not focusing on what the scoreboard says, but trying to implement our brand and our footy," Longmuir said.
"We know that starts in the contest, and I thought we were inconsistent in the contest in the first half.
"Led by our leaders, we were much more consistent in the contest in the second half.
"We put our name on 50-50 balls, we owned the footy, which gave us territory, and we defended our front half a bit better.
"We were able to score off the back of those things rather than just ball movement."
The result gave Fremantle (3-2) a positive record for the first time this season ahead of an away clash with winless Melbourne in round six.
Richmond (1-4) have lost four straight going into their meeting with Damien Hardwick's Gold Coast.
Jacob Hopper (33 disposals, five clearances) fought hard for an inexperienced Tigers line-up against Fremantle, and Maurice Rioli Jnr kicked two goals.
But coach Adem Yze conceded his side never created enough scoring opportunities, trailing in the inside-50 count 32-19 at halftime and 65-36 at the final siren.
The Dockers piled on 6.1 to 1.0 in the third quarter to rip the game away from their opponents.
"We felt like we were in the game at halftime, but we didn't have the volume of entry that we'd like," Yze said.
"That was sort of a red flag at halftime.
"It's pretty hard to stay in the game when you're getting smashed in territory.
"The second half just wasn't to standard, and we've had a couple of really ordinary third quarters this season, which we need to look at."
