As revealed by Autosport last month, Holdsworth was quickly snapped up by the Holden squad after losing his full-time drive with Tickford Racing at the end of last season.
Warren Luff, who partnered Mostert to third at the Great Race last year, will move across to Bryce Fullwood’s #2 entry.
The WAU/Holdsworth deal comes after the three-time race winner took provisional pole at Mount Panorama last October, and marks a return to the Walkinshaw fold after driving a Team 18 customer entry back in 2015.
The limited Supercars programme for 2021 will dovetail with Holdsworth’s full TCR Australia campaign, which got off to a winning start at Symmons Plains last week.
“I’m really pleased to be in the Walkinshaw Andretti United camp this year, they’ve been building up for some time now, and you can really see the progression,” said Holdsworth.
“I’m happy to be a part of that building process, and 2021 looks like it’s shaping up to be a great year for the team, especially seeing the way they finished 2020.
“It’s exciting to be on board. Chaz is one of the very best, WAU is on the move, and the Mobil 1 Appliances Online Racing #25 looks brilliant. I’m looking forward to having my first steer of the car.
“Chaz and I are good mates. we know each other well and we’ve always had respect for each other both on and off the track.
“It’s going to be completely different for me this year, I’ll be in the car while Chaz is having his lunch break, but I’ve got a big job to do.
“I think the car will be great, which is one thing, but I’ll be enjoying every lap I do at Bathurst. I will do my best to help the team bring home the goods.
“It’s nice to come back into the workshop and see a lot of familiar faces. There’s great continuity, a lot of passion, and a great history amongst the team.”
Luff, meanwhile, will be looking to add to his run of four podiums in the last six years for WAU.
“It’s fantastic to be back in the Walkinshaw Andretti United family for another season, we’ve had some great moments together over the last seven years, and I’m hoping to add to that at Bathurst this year,” said Luff.
“Bryce really showed he belongs in this category in his first season, I think he surprised a few people with his speed and craft, and the ‘rookie’ tag wasn’t reflective of his performances, so I’m really excited to be sharing the car with him at this year’s Bathurst 1000 in the Mobil 1 Middy’s Racing #2.
“With just the one endurance race this year, being comfortable with the team, and the car is going to be really important come October, so I’m looking forward to jumping behind the wheel at Winton, and getting the year underway.”
Both Luff and Holdsworth will join WAU for the pre-season test at Winton on 16 February.