Mostert, who led a front row lockout for Walkinshaw Andretti United in qualifying, followed his team-mate Ryan Wood for the opening 14 laps of the 55-lap race before taking the lead and pacing himself to a 4.9-second win over Triple Eight's Will Brown.
Ford's first win, in the ninth race of the season, was determined by pit strategy as well as speed.
Brown took four tyres at his mandatory pitstop and four laps later, Mostert took only two. That gave Mostert a gap of five seconds, which he managed to hold to the end of the race.
"[We are] just being aggressive, you know? Last year, we didn't quite feel where we were with the window with the car," said Mostert after his first win in nearly two years.
"We tried a lot of things. This year, I felt like every time he touched the car, it either did something. So [the recent] test day was about trying some big philosophy changes and we came here with it. We had to try and piece it all together and make it work. For [our] first crack at it, I think it's pretty good."
Brown, who managed to stretch his series lead to over 100 points with second place, looked content with the result after starting from fourth.
"Once we got past Ryan [Wood], I probably struggled a bit with my tyres and Chaz was still fast," he said.
"He [Wood] was doing the same times with two tyres, so we got a bit of work to do overnight, but still another second place. Good for the championship."
Third place was decided after a huge fight between the Fords of Wood and Tickford's Cam Waters. Wood, 20, drove superbly and Waters only managed to snatch the position on the final lap of the race.
In spite of losing the podium spot, Wood's performance suggests that WAU appears to have, for now at least, overcome its recent struggles at one of its bogey circuits in a convincing fashion.
Triple Eight's Broc Feeney finished fifth ahead of veteran James Courtney (Blanchard Racing Team Ford) and Matt Stone Racing's Nick Percat (Chevrolet).
The day was a disaster for Erebus Motorsport, with Jack Le Brocq suffering a battery failure on the grid and defending champion Brodie Kostecki running off on the first corner, dropping to last. He recovered to 10th by the end of the race, on a home circuit on which he starred a year ago.
Likewise, Grove Racing's Richie Stanaway was out early when Aaron's Love Ford immediately in front of him experienced a fuel pump stutter. Stanaway hit him from behind, hit the wall and pitted before resuming, finishing 24th and last, 11 laps down.
The results stretch Brown's points lead to 103 (952-849) over team-mate Feeney, with Mostert in third on 763. Percat and Grove Racing's Matt Payne are tied for fourth on 606.
Supercars Perth - Race 1 result
| Cla | Driver | Car / Engine | Delay |
| 1 | Chaz Mostert | Ford | |
| 2 | William Brown | Chevrolet | 4.9134 |
| 3 | Cameron Waters | Ford | 13.1297 |
| 4 | Ryan Wood | Ford | 13.4661 |
| 5 | Broc Feeney | Chevrolet | 17.8079 |
| 6 | James Courtney | Ford | 21.7942 |
| 7 | Nick Percat | Chevrolet | 25.6341 |
| 8 | Matthew Payne | Ford | 26.0108 |
| 9 | A.De Pasquale | Ford | 30.2070 |
| 10 | Tom Randle | Ford | 31.9240 |
| 11 | Cameron Hill | Chevrolet | 32.6170 |
| 12 | Tim Slade | Chevrolet | 33.1726 |
| 13 | A.Heimgartner | Chevrolet | 33.4673 |
| 14 | James Golding | Chevrolet | 34.3121 |
| 15 | Bryce Fullwood | Chevrolet | 37.7807 |
| 16 | M.Winterbottom | Chevrolet | 41.9684 |
| 17 | David Reynolds | Chevrolet | 43.5845 |
| 18 | Jaxon Evans | Chevrolet | 44.4463 |
| 19 | Macauley Jones | Chevrolet | 44.8055 |
| 20 | Jack Le Brocq | Chevrolet | 47.3078 |
| 21 | Aaron Love | Ford | 1'14.2225 |
| 22 | Brodie Kostecki | Chevrolet | |
| 23 | Will Davison | Ford | |
| 24 | Richie Stanaway | Ford |
