Winger Josh Martin produced a stunning aerial effort to score, before Thirroul repelled Wests late charge in Saturday's gripping Illawarra President's Cup clash at Parrish Park.
Martin leapt to gather in playmaker Hayden Crosland's crossfield kick and finished upside down as he planted the ball inside the in-goal, setting up the 20-14 triumph.
The try took Thirroul to a 12-2 half-time and hooker Jye Bowen's determined dummy-half run in the 68th minute, making it 18-8, looked to have sealed victory for the Butchers.
But as they did with Brock Sepulveda's catch to score off Callum Waldock's kick, Wests found a way back into the contest when Mitch Porter's pass under pressure found Mitchell Phillips to dive over out wide with six minutes remaining.
Still, Thirroul held on.
"We had to turn them away a couple of times at the end, it was a tough day," coach Jarrod Costello said.
"We made some errors there in the second half, but we just kept fighting for the win. That's a great try from Josh Martin, to get up high and still put it down."
It moves Thirroul clear of Wests in second spot on the ladder, only behind unbeaten Collegians - who had the bye - on for-and-against.
Key injuries have meant Costello has had to change halves and backline combinations in the opening rounds, but Crossland played a key role after switching from fullback to five-eighth.
"He's just one of those players, he's a Mr Fix It for us," Costello said.
"We've had HIA (Head Injury Assessment players) and injuries worries and just haven't been able to settle at all.
"But I think Crossy is still underrated to some people, he's moved into the halves and done a great job. Hopefully we'll get (hooker) Ryan Fletcher and (prop) Tim Grant back soon as well and we can start to build into our season."
Meanwhile, Corrimal launched early blitzes in each half to setup a 24-16 win over Dapto at Ziems Park.
The Cougars surged to a 14-0 lead in the opening 26 minutes through tries to Harry Rudd, Tongo Tongotongo and Liam Scott.
A intercept led to winger Tom Gaffey scoring late, but it was too late for Dapto.