The French outfit had won eight successive home games against the Tigers but their hopes of making it nine were over in the early stages of the second half.
Two tries each for Denny Solomona, Paddy Flynn and Ben Crooks gave Castleford a 34-10 lead that was enough despite a late Catalans fightback.
Olivier Elima, Glenn Stewart, Eloi Pelissier, Louis Anderson and Richie Myler all crossed but the 34-28 defeat compounded a miserable second half to the season for the Dragons.
“To come out on top is fantastic for us,” said Castleford coach Powell.
“We’ve got a very young team out there today with a lot of experienced players sat on the sidelines with injuries.
“I’m delighted with the win. There were times there where we could have been better with our game management towards the end but overall we’re pleased to pick up the two points.”
Solomona’s double took him to 37 tries for the season and saw him break Lesley Vainikolo’s 12-year Super League record.
“Clearly it takes a lot of doing,” added Powell.
“He’s been on the end of some outstanding left-edge plays and I thought Luke Gale gift-wrapped a couple of tries for him tonight.
“Obviously he’s had some great finishes this year but he’s had some great service too.”
Castleford are now only one point behind Catalans in the Super 8s table and Powell hopes to continue building momentum they can take into 2017.
“That is the plan,” he said. “When the Super 8s started we were looking to try and catch St Helens, but that wasn’t to be.
“But we’ve got momentum because there is belief running through the team because we know wherever we go and play, our style of play and our brand of rugby league is capable of causing teams a lot of trouble.”
Catalans were second in Super League after beating St Helens in June to raise hopes of a first Grand Final appearance, but the issues they have had since were evident in their latest defeat.
“The pleasing thing is we managed to come back hard and try hard but I’m not happy with the way we defended,” said coach Laurent Frayssinous.
“We knew Castleford would be very good in attack but in the first half we couldn’t control the ruck and we made bad decisions on the edges.
“Defence cost us the game. Even if we tried hard to get back into the game, I’m disappointed with the way we defended.
“I thought we were in the game and competed in the first 20 minutes but after that a lack of execution cost us the three tries when we couldn’t take the short kick-off.
“We had a game plan defensively to stop them but unfortunately that didn’t work, which is disappointing.”
Catalans Dragons: Gigot, Sigismeau, Garcia, Duport, Broughton, Stewart, Myler, Anderson, Mounis, Horo, Elima, Aiton, Casty. Substitutes: Taylor, Bousquet, Pelissier, Seguier.
Castleford Tigers: Dorn, Flynn, Minikin, Webster, Solomona, Hampshire, Gale, Patrick, McShane, Cook, Savelio, O. Holmes, Milner. Substitutes: Crooks, McMeeken, Maher, Douglas.
Referee: Phil Bentham (RFL)