It's a landmark achievement for WAU, who swooped on Mostert's services ahead of 2020 but faced apparent pressure from Triple Eight to keep him going forward.
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“We are delighted to continue our journey with Chaz,” said Ryan Walkinshaw, who is currently in Australia.
“He has been instrumental in our development and the path we are on as a team, we definitely have big things we still want to achieve together, which was always the goal when we started.
“He is without-doubt one of the leading drivers in the category, and we have worked really hard as a collective to put ourselves in the fight.
“It's not just on the track we are seeing results though, he is a major part of our family, and fits into our culture perfectly.
“We remain completely focussed on the 2021 season, but it's nice to know our future with Chaz is now secured, and I can't wait to see what we can continue to achieve.”
“Having Chaz remain with Walkinshaw Andretti United into the future is so exciting,” he said.
“He is a world-class driver who has been instrumental in our growth as a team. We share the same ambitions, and want to finish what we started together.
“We are in a really strong position in 2021 thanks the strong start to the season, so I can't wait to see what we can achieve not only in 2021, but into 2022 and beyond.”
Added Michael Andretti: “We are excited to announce the re-signing of Chaz with our team, he is a fantastic driver and a huge part of our family.
“When Chaz joined Walkinshaw Andretti United in 2020, we had big ambitions we wanted to achieve together, and today's news is another step toward that.
“If 2021 to date is anything to go by, I think we have a lot to look forward to together, and I can't wait to see it unfold.”
Mostert and team-mate Bryce Fullwood will be on track this weekend for Round 5 of the Repco Supercars Championship, the Merlin Darwin Triple Crown.