A year after losing his mum Dening to cancer and tumbling outside the top 100, Day out-duelled world No.2 Scottie Scheffler, then fended off raft of other challengers in a tense Sunday shootout at TPC Craig Ranch.
After seeing off Scheffler, Day signed for a brilliant bogey-free final-round nine-under-par 62 to secure a steely one-shot win over Si Woo Kim (63) and American Austin Ekroat (65).
Kim's Korean countryman CT Pan (62) was fourth at 21 under, with Scheffler (65) ultimately finishing fifth three shots back after being unable to stay with Day down the stretch.
Kim birdied the last to briefly join Day atop the leaderboard but the Queenslander held his nerve to respond and deliver his wife Ellie - greenside and heavily pregnant with the couple's fifth child - her own priceless Mother's Day gift.
In a poignant tribute, Day's caddy sported a bib with his mother's birth name "Adenil" and the inspired former world No.1 honoured her memory in style with his first tournament win since the 2018 Farmers Insurance Open.
"This was the first one (Mother's Day) kind of without her so to have her on my caddy bib was special," Day said.
"I was in tears for a little bit there, and to think about what my mum went through from 2017 on to her passing last year and then to know that.
"It was very emotional to go through and to experience what she was going through, then I had injuries on top of all of that going on in my life.
"And then obviously Ellie, the amount of sacrifices she's made for me and my career, I just can't thank her enough.
"She never gave up on me trying to get back to the winner's circle again. She just always was pushing me to try and get better.
"To be honest, I was very close to calling it quits. I never told my wife that, but I was okay with it, just because it was a very stressful part of my life."
In a huge confidence booster ahead of next week's US PGA Championship, the season's second major in New York, Day finished at 23 under and had only three bogeys for the week.
Fittingly, the 35-year-old ended his barren run at the same tournament where he clinched his maiden PGA Tour win in 2010.
But his 13th triumph is even more significant and completed Day's tumultuous journey back from heartache and despair.
He was languishing at 175th in the world last September before rebuilding his swing to combat the debilitating back injury that threatened to prematurely end a once-great career.
But the 2016 PGA Championship winner's resilience is now paying off, with Day among the hottest players on the planet once again.
He posted six top-10 finishes this year and eight consecutive top-20s before an untimely bout of vertigo ruined his Masters' chances.
As well as the trophy, he pocketed a cheque for $US1.7 million ($A2.6m) after snapping his 1835-day title drought.
"It's been a struggling few years so to be able to get the win the way I played today was really special," Day said.
"I've had a lot of injuries with my back and for a moment there I thought I wasn't going to play again and then just trying to get through those two years and just trying to get through a tournament was difficult.
"So to be on the other side of it, being healthy, feeling good about my game, finally winning again, there's no better feeling really."
Compatriot Adam Scott briefly enjoyed the clubhouse at 19 under following an eight-birdie final-round 63 before eventually settling for equal seventh.
Aaron Baddeley (68) tied for 23rd at 15 under, while David Micheluzzi (70) finished at nine under in a respectable share of 67th on his PGA Tour debut.
