The win in Bathurst halts the Tigers' losing streak after seven losses to begin the season and five to end 2022.
Penrith were stilted and error-prone in torrential rain on Saturday night but mostly fell victim to a polished Tigers side that finally delivered on the potential shown this month.
"It's not just a win, it's a really good win," Tigers coach Tim Sheens said.
"There was no luck in it. We worked really, really hard.
"We had a good game plan we thought, we prepared for the rain, and the boys carried it out almost to the letter.
"That will show the guys that if they stick to what they're prepared to do, we've got a decent football team there."
The result intensifies pressure on an admittedly undermanned Panthers side that has now started their premiership defence with four wins and four losses.
"In the conditions, they played better," Panthers coach Ivan Cleary said.
"They deserve a lot of credit but we didn't play real smart tonight.
"We definitely feel like we added to the loss ourselves tonight."
Without injured halves partner Adam Doueihi, Brooks rose to the challenge of booting the Tigers into momentum.
The Tigers' two tries both came directly after Brooks 40/20 kicks, the first of which was in the first set of the night.
After his first pinpoint kick, Brooks threaded a grubberkick through the defence to give Brandon Wakeham the opening try - all before the reigning premiers had touched the ball.
"His kicking game was how we won today," Koroisau said of Brooks.
Hooker Koroisau had his best game since leaving the Panthers last year and was the Tigers' general in the middle of the park.
He came within inches of scoring a try from his own grubberkick in the first half and just after halftime, tore through the defence on a line break that could have given the Tigers a second try had there been a support runner.
The performance would not have gone unnoticed by NSW coach Brad Fittler, who is set to choose between Koroisau and Damien Cook when he names his team for State of Origin I.
But penalties cost the Tigers; they conceded three in the space of eight minutes in the first half and on the back of a high tackle from Brooks, the Panthers marched upfield to score their only try through Tyrone Peachey.
The Tigers did well to avoid conceding points while centre Brent Naden was in the sin bin for laying a shoulder charge on Zac Hosking that is likely to earn him a suspension.
"When 'Nado' went off to the bin we were awful," Cleary said.
In his second NRL game, Tigers fullback Jahream Bula was outstanding, setting David Nofoaluma up for his 100th career try with a cut-out pass and fielding Nathan Cleary's high kicks with aplomb all night.
Down 12-8, Penrith looked to have equalised when Nathan Cleary burst through the defence but the ball bounced free of his grasp in slippery conditions as rookie fullback Jahream Bula slid over in cover defence.
It was one of 11 errors for the Panthers, many of which came in good territory.
Referee Adam Gee pulled the play back to sin-bin Penrith interchange hooker Soni Luke for a hip-drop tackle on Alex Twal as the final 10 minutes approached and the Tigers held on.
Injury continues to ravage the Panthers' middle forward rotation, with Matt Eisenhuth set to join James Fisher-Harris and Spencer Leniu on the sidelines after picking up a game-ending shoulder complaint in the 15th minute.
Cleary was unsure of the severity of Eisenhuth's injury post-match.
