The Australian duo were drafted in as substitutes for Callum MacLeod and Mike Price, who were ruled out by sailing commitments.
Darren Leung and Dan Harper took pole position and controlled the opening two hours of the race in their Paradine Motorsport BMW M4.
CLICK HERE for Silverstone 500 results
However, a call to go off-strategy combined with some well-timed Safety Cars brought Shahin and Patterson into contention.
From ninth on the grid, they steadily worked their way forward.
In a race riddled with yellow flag and Safety Car interventions, the decisive moment of the race came in the final hour when a well-timed caution coincided with their final compulsory pit stop.
Meanwhile, their rivals had to pit under the green flag and were promoted to the race lead.
The win came despite an engine issue that Patterson and Shahin had to manage for the duration of the three-hour affair.
“What a day,” said Patterson.
“We struggled all day with the car, to be fair. We had a bit of a mechanical issue we were driving around.
“Just had a little bit of an engine issue that was playing up that we had to manage.
“The pace wasn’t there but we’re smart, we’ve done enough racing.
“Brains normally win most races around the world, so I think we managed it pretty good.
“To get the win in the Silverstone 500 with what we’ve had today is pretty mega.
“I guess we got a little bit lucky with the strategy but you create your own luck, so we’re happy.”
It’s the first time since 1997 that any Australian driver has won a British GT Championship race – the last being Charlie Cox at Croft driving a Harrier LR9 GT97.
“I’ve learned not to get too excited too early,” said Shahin.
“Anything can happen in the last few minutes. We were getting excited at the 35-minute mark and then the Safety Car came out and then we were just hoping that the car would be fine.
“All’s well that ends well.”
Australia’s other interest, Josh Buchan and Cameron Campbell, failed to finish after their Rodin Motorsport Ferrari 296 suffered damage after contact with a slower GT4 car.
Discussion about this post