He racked up 30 possessions and snapped an outstanding goal as the Lions mauled Sydney by 60 points on Saturday at the MCG.
Ashcroft polled 14 votes to win ahead of Lions co-captain Lachie Neale (eight), while teammate Callum Ah Chee had seven and Kai Lohmann one.
The 20-year-old is the youngest Norm Smith medallist since Carlton's Wayne Harmes was crowned the inaugural winner at age 19 in 1979.
"I can't believe it," Ashcroft told the Seven Network, having capped his comeback from an ACL injury in sublime fashion.
"As hard as it was to miss last year, I set myself a goal.
"I set myself a goal in the second half of the year and to come back and play my first finals (series).
"To be here, and then win it, my god, unbelievable."
Ashcroft out of traffic ⭐️— AFL (@AFL) #AFLGF pic.twitter.com/11IYbh7SInSeptember 28, 2024
Ashcroft is the son of Marcus Ashcroft, who played in Brisbane's last premierships, the 2001-03 three-peat.
He was recruited by the Lions as a father-son pick and made an immediate impression on debut last year, only to suffer a serious knee injury just before the finals that meant a reconstruction.
Ashcroft returned late this season to play a pivotal role in Brisbane's march to the flag.
Marcus and wife Bekky were in the crowd on Saturday along with middle son Levi, who is also about to go to the Lions as a father-son selection.
Youngest daughter Lucy, 15, is a state-level footballer and netballer, too.
Several Lions had staked claims for best afield, with Neale racking a game-high 34 possessions and nine clearances.
Once Brisbane went on their second-quarter rampage, the field narrowed considerably.
Sydney star Isaac Heeney was the popular pre-game choice, but he endured a horrible day and limped off with a leg injury in the third term.
Neale was massive from the start, so clean as always at the coalface and racking up clearances.
Lions midfielder Lachie Neale with one of his game-high 34 possessions against Sydney. (James Ross/AAP PHOTOS)
Coach Chris Fagan had said before last Monday's Brownlow count that winning a third medal probably was not on the top of Neale's priorities.
As it happened, Neale's grand final week became much more straightforward, unlike last year when he won his second Brownlow.
And after grand final losses at Fremantle in 2013 and last year's epic clash with Collingwood, Neale is now a premiership player.
Andrews was also mighty, again performing his role to perfection with intercept marks across half-back, while Ah Chee kicked four goals and Lohmann lit up the game with three goals in the first half.
Meanwhile, amid rumours that this is his last AFL game, Joe Daniher had a "nearly" game.
He was certainly among Brisbane's best, and sparked wild celebrations with his last-quarter goal, but had Daniher kicked straighter than his 2.4, he might have added a Norm Smith Medal to his family's storied AFL history.
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