However "Swimming" was ultimately beat by Cardi B's "Invasion Of Privacy."
Mac's mother, Karen Meyers, said she would be present at the ceremony on her son's behalf, ready to accept the honor had it fallen to her.
Karen and her husband Mark had previously revealed that if their boy missed out, they were rooting for Cardi to win.
Not everyone was happy with The Recording Academy's choice however; the Self Care hitmaker's ex-girlfriend Ariana Grande reacted angrily, tweeting and swiftly deleting "f--k", "trash" and "literal bullshit" immediately after the announcement.
When a fan insisted she wasn't shading Cardi, Ariana confirmed: "nothing to do w her. good for her. i promise. i'm sorry."
When another fan suggested Cardi was "trash tho', Ariana interjected 'she's not at all and that's not what i meant and u know that'.
A third fan posted a news article linking Ariana's tweet to Cardi's win, she replied in all caps: "THEY INVITED HIS PARENTS OUT. NO."
Grande had herself earlier nabbed one of the two awards she was up for — Best Pop Vocal Album for "Sweetener" — following her very public dispute with a Grammy producer over what songs she could perform, resulting in her pulling out of the show. She missed out on the Best Pop Solo Performance award, her "God Is A Woman" losing out to Lady Gaga's "Joanne (Where Do You Think You're Goin'?)"
Meanwhile an extremely excited Cardi announced in an Instagram video backstage she was sharing her award with Miller.
"And you wanna know something, I read an article that Mac Miller family said that if he don't win, they want me to win, so I'm sharing this Grammy with you motherf**ker! Rest in peace!"
"Swimming," which peaked at number 3 on the Billboard charts, was up against a strong line-up on the night which also included Travis Scott's "Astroworld", Nipsey Hussle's "Victory Lap" and Pusha T's "Daytona".
The nomination was the first for Mac — born Malcolm James McCormick — and would likely have not been the last for the rising talent, had his life not been tragically cut so short.
Since Mac's death, his parents have been keeping their son's legacy alive with The Mac Miller Circles Fund, which has already raised upwards of $1 million in donations to help children in his hometown of Pittsburgh.
Sadly Mac wasn't the only posthumous nominee on the night: Chris Cornell, who committed suicide in May of 2017, won Best Rock Performance for “When Bad Does Good” His 14-year-old daughter Toni and 13-year-old son Christopher accepted the award on their father's behalf.
“It was very difficult because, obviously, we miss him so much and we saw him work on this so hard – he was always working on music, it was his passion,” Toni said backstage.
“It was really sad in a way to feel like he couldn't be there himself to accept it for something that he was so proud of and worked so hard on. Again, we're so proud of him and it was amazing.”
Chris joins an illustrious company of posthumous winners, including David Bowie, Amy Winehouse, George Carlin and Joan Rivers, as well as Ray Charles who landed eight Grammys in 2005, the year after he died.