Mostert improved with his final run but ended up four tenths back in second, ahead of Shane van Gisbergen, who had a trio of offs during the 40-minute session.
"I was so nervous before that qualifying session because we had such a good car and this is probably going to be the only wet session that we have," said McLaughlin.
"I'm really proud. We've been struggling with our wet car, but we had an awesome car from the start. That was hands-down the best wet car I've ever had."
Added Mostert: "The second set [of tyres] we put on didn't come in until very, very late. It is what it is. At least we're in the 10.
"Everyone in pitlane can give themselves a pat on the back. I had about 10 or a dozen moments out there were I thought 'that was close', no doubt everyone was feeling that."
Will Davison was fourth, ahead of Brad Jones Racing teammates Tim Slade and Nick Percat.
Jamie Whincup and Cameron Waters looked set to miss the cut for the Top 10 Shootout today but made it through in seventh and eighth.
Whincup battled for pace for much of the session after a practice five crash, while Waters only made it out on track in the final 10 minutes after battling sensor issues.
"We probably didn't change enough for the wet and we were down in the 20s," noted Whincup.
"But the engineers did a good job, they ground away at it and just scraped into the 10, which is nice."
Mark Winterbottom and Anton De Pasquale snuck through in the 10, with Rick Kelly the unlucky man to miss out as the lead Nissan.
Fabian Coulthard (16th) and Lee Holdsworth (20th) were the biggest scalps, leaving themselves with a big mission in tomorrow's race.
Two 30-minute practice sessions take place today ahead of the Top 10 Shootout.
