The Dragons dominated from the outset at Suncorp Stadium in their first victory of the year after losing 11 on the trot in 2026.
It was their first success in 295 days, dating back to a 22-14 result over Cronulla on August 9 last year.
The loss was the bumbling Broncos' fourth in a row and has left their premiership defence in tatters, despite a powerhouse display by prop Payne Haas, who carried the team on his back in his return from injury.
To compound their woes, Queensland forward Pat Carrigan succumbed to an ankle injury.
St George Illawarra's victory showcased the power and passion of a young forward pack that is the future of the club.
Centre Valentine Holmes had his best game of the season and halves Daniel Atkinson and Kyle Flanagan combined like never before.
When the Dragons led 30-8, Broncos coach Michael Maguire stormed out of the coaching box and went to the sideline.
Brisbane hit back with three tries but the Dragons hung on to give interim coach Dean Young his first win.
The visitors shocked the Broncos to take a 14-2 lead into halftime. The young Dragons forwards, to a man, were superb in the opening exchanges, with prop Toby Couchman leading the way.
Second-rower Dylan Egan did well to strip the ball from Carrigan. Fellow back-rower Hamish Stewart showcased his footwork to send Holmes in.
A lovely grubber kick for himself by five-eighth Daniel Atkinson stunned the hosts further.
Broncos second-rower Brendan Piakura was put on report for a late hit on Atkinson and the Holmes penalty had the Dragons out to 14-0.
Walsh was off his game in the first half, with one kick gifting the Dragons a seven-tackle set and a loose pass without looking coming as another glitch.
He was also caught napping when Atkinson put in his crafty kick.
The Dragons appeared likely to score the first try of the second half but a Clint Gutherson pass went straight into the breadbasket of Jesse Arthars, who raced 90m to score.
Young's men would not be denied and twins Toby and Ryan Couchman stormed up the field. Winger Max Feagai got on the back of a cross-field kick by Atkinson to take the lead back to 12.
Some Holmes magic sent the Dragons fans into delirium when he latched onto an Adam Reynolds pass, firstly with his fingertips before kicking ahead to score under the sticks.
When Setu Tu scored in the 61st minute the Dragons had a 30-8 lead.
Brisbane came back with Walsh putting winger Josiah Karapani over out wide. Giant forward Xavier Willison stormed over off a Ben Hunt pass to narrow the gap to 10.
Walsh then sent Arthars in for his second but it was too little too late, as the Dragons held on for a deserved win.
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