Coulthard, making a second cameo start in the series in a Wall Racing Honda, set up a maiden TCR victory with a rapid first three laps.
He slipped into third place on the opening lap before charging under Ben Bargwanna (Garry Rogers Motorsport Peugeot) to grab second on Lap 2.
The Kiwi then jumped early leader Michael Clemente (Michael Clemente Motorsports Honda) on Lap 3 to take a lead he would hold for the remainder of the 16-lap journey.
Coulthard was never quite able to entirely shake Bargwanna, who ended up back in second when Clemente had an engine problem, but looked to be managing the gap as he won by half a second.
"It's great," said Coulthard, who missed out on a full-time Supercars seat for the first time since 2007 this year.
"We had a fantastic start, we were able to pick a few off and then we got Bargwanna.
"It's an awesome day for the team. I can't ask for much more. We're always working hard, you can always improve the car and make it better. This is a good start, we've just got to keep the momentum going."
Five cars found themselves in the battle for third in the closing stages of the race.
In the end the place went to Tony D'Alberto, capping off a good day for the Wall Racing squad, although only after rubbing panels with Jordan Cox (GRM Peugeot) a few times.
In the end D'Alberto had Cox, Dylan O'Keeffe (GRM Peugeot) and Melbourne Performance Centre Audi teammates Jay Hanson and Will Brown in his mirrors.
Michael Caruso (Ash Seward Motorsport Alfa) came from near the back of the field to finish eighth place ahead of Hyundai pair Nathan Morcom and Brad Shiels.
Zac Soutar (Team Soutar Motorsport Honda), who went into the race as the series leader, had a tough outing, starting with a whack from Bailey Sweeny (HMO Hyundai) while scrapping for ninth on Lap 7.
That sent him off the road at the exit of MG, before another off at Turn 1 later in the race left him down in 18th.
Aaron Cameron, meanwhile, looked quick in the early laps of the race, only for turbo to fail on his GRM Peugeot.
The final TCR race of the weekend will start at 4:10pm local time.
Watch it live and free on Motorsport.tv.
