The Supercars regular was untouchable in the wet conditions, quickly breaking away from the pack before setting a dominant pace across the 15-lap journey.
He ended up 32s clear of the field, with a best lap more than two seconds better than anyone else.
“It was good fun,” said Mostert. “It’s probably one of the best wet cars I’ve ever driven; the tyre gave me great feedback, I could mix the lines up and I felt like I was getting faster and faster. I had an absolute ball out there.”
Luke King made it a Melbourne Performance Centre Audi one-two, but didn’t have things quite as easy as Mostert. He was jumped by Lee Holdsworth at the start, before passing the ASM Alfa at MG corner on Lap 5.
Once back in second King had little problem pulling away from Holdsworth, his advantage a healthy 8s by the finish.
Holdsworth came home a lonely third which helped him into the points lead, while a significantly improved showing from the Hondas saw John Martin finish fourth and fifth.
Aaron Cameron finished sixth in his GRM Peugeot ahead of HMO Hyundai driver Josh Buchan who survived hair-raising off on the run to Honda mid-way through the race.
Father/son duo Jason and Ben Bargwanna finished eighth and ninth in their GRM Peugeots, as Lachlan Mineef (MPC VW) rounded out the Top 10 on his TCR debut.
The second round of the 2021 TCR Australia season continues with two more races tomorrow.
You can watch all the action from Phillip Island live and free on Motorsport.tv.