A field goal by half Jock Madden in the 76th minute sealed the deal on a night where Sea Eagles prop Matt Lodge ruptured his biceps.
With regular fullback Reece Walsh out with injury, Cobbo switched from centre to No.1 on Friday night and was in scintillating form in front of a sold-out Magic Round crowd of 50,971 at Suncorp Stadium.
The 21-year-old made three line-breaks and 11 tackle busts and ignited the Broncos attack from the back with his power and speed.
His positional play was also first-class.
Selwyn Cobbo was in brilliant form for Brisbane in their 13-12 victory. (Dave Hunt/AAP PHOTOS)
"He only found out yesterday after training he was playing fullback. It's a totally different role to centre obviously, but he did some really good stuff at the back," coach Kevin Walters said.
"He doesn't care what the score is, he just plays Selwyn so I thought he was really good for us - particularly some of his carries and also the high balls he caught there at critical times."
Cobbo has represented the Maroons in State of Origin on the wing, where he played all of his club football in 2023.
The Broncos switched him to centre this year and he has made a seamless transition.
Cobbo's proven versatility makes him a more appealing prospect in Origin. He played the first game of last year's series, before a hip injury kept him out of the rest of the campaign.
The Sea Eagles had 65 per cent of the territory in the first half, when Brisbane's goal-line defence was superb.
The Broncos led 12-0 at halftime, but Manly hit back when centre Reuben Garrick scored after what appeared to be a forward pass from back-rower Haumole Olakau'atu.
When giant winger Raymond Tuiamalo Vaega crashed over it was 12-all before Madden stepped up.
"We've got a lot of confidence in Jock and what he brings to the team. He's a different type of player to Adam Reynolds, but tonight we saw a cool head in a tough period," Walters said.
Brisbane prop Payne Haas limped off with an injury to his left big toe and was assisted from the field in the 60th minute when the Broncos led 12-6. He returned 14 minutes later in a massive boost to the NSW Blues.
Earlier, Madden threw a delightful pass for second-rower Brendan Piakura to hit a hole and score.
The Sea Eagles lost prop Taniela Paseka to the sin bin and the Broncos made them pay.
It was his halves partner Ezra Mam who stepped up with a skilful double-pump to send electric winger Deine Mariner over.
Blake Mozer, Brisbane's highly rated 20-year-old hooker, came off the bench in the 34th minute and put two massive hits on Sea Eagles forwards Matt Lodge and Ben Trbojevic, which dislodged the ball.
Manly coaach Anthony Seibold said he was "really proud of the guys"
"I thought they were really tough," he said.
"There was a real hard edge of how we played our game, and probably Tolu Koula was a classic example. He got a really nasty knock to his AC joint but stayed on the field and kept fighting really hard."
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