Mackellar was the only rider from the peloton to successfully chase down a six-man breakaway with two laps to go, before dropping the group and powering home unchallenged to the finish line in Buninyong.
Brady Gilmore (ARA Skip Capital) and Alex Bogna (Alpecin-Deceunick) took the minor placings with second and third respectively as they finished in the break a minute and five seconds behind Mackellar.
"It felt unreal, I've been dreaming of that for years. To finally be able to do it is so surreal," Mackellar said of the win after the finish.
"I've raced this course for a couple of years now. I was a bit worried when the break got out to three-plus minutes but I knew it's always the last one or two laps that win this race in the past and I knew my legs were good.
"I just kept telling myself I needed patience, had the legs with two laps to go and I'm surprised I did it."
Attacks flowed on Mount Buninyong as the race picked up on the fourth lap of the twelve lap circuit, a 10-man breakaway establishing which would come to define the day's racing.
Initially working well together, the break stretched out a lead of as much as three and a half minutes on the trailing peloton, riders failing to get together and put up an effective chase as laps went by.
The pace in the bunch picked up going into the final few laps with riders falling off the back and the peloton stretching to single file on the road as Mackellar surged to the front and took up the pacing.
That move cut the break's advantage from three minutes and 20 seconds to two minutes and 40 seconds into the final four laps as up ahead the leading group was becoming uncooperative.
Mackellar then went from the peloton again with Gilmore and Dylan Hopkins (Ljubljana Gusto Santic), the latter pair didn't stick around for long as the pace of the 20-year-old set a blistering tempo.
The remnants of the attrition-diminished break were up next, before Mackellar caught the leading group on a dip in the road through Federation University and surged right over the top of them to complete what had been a long and fantastic move to get to the front of the race.
The break managed to respond, but were visibly struggling after a long day in the saddle while Mackellar had speed to burn in the last lap with another move on the final ascent of Mt Buninyong that none could follow.
Surging ahead solo, Mackellar outlasted all other riders throughout the final lap and sailed home a minute ahead of the chasing group to complete one of the biggest races and results of his young career.
The 2023 Australian Road National Championships continues tomorrow with a super Sunday featuring the Elite Men's and Women's Road Races. Watch all the action from 10:30am AEDT LIVE on SBS and SBS On Demand.
