The home team turned things around quickly in the second half, scoring three exceptional goals in the first 21 minutes after the break.
Australian Sam Kerr levelled things at 2-2 straight after the interval with a scorching volley, demonstrating why she was recently named Women's Footballer of the Year by England's Football Writers' Association.
In the 51st minute, Chelsea took the lead for the first time, courtesy of Guro Reiten's strike near the penalty spot. Kerr soon reclaimed the spotlight with a strike similar to her first – the golden boot winner's 20th goal of the season.
"The second one was world-class; it was outrageous, audacious," said Hayes.
"She's the best for a reason and she stepped up once again for this football club."
Kerr said after the game: "I'm not known for my 'worldie' goals but the thing that I'm known for is getting it done when it matters."
In the early stages of the match, the visitors had struck first with Martha Thomas finishing a lethal free kick delivered by Katie Zelma in the 13th minute.
Erin Cuthbert then equalised for Chelsea, drilling a strike into the bottom corner.
United took back the lead courtesy of Ella Toone on the end of Leah Glaton's cross into the box.
Then came Hayes' halftime regrouping in the sheds that changed the title race.
"First half we played with fear," Hayes said. "Second half we played to win."
Hayes introduced South Korea midfielder Ji So-yun, in her final home game for Chelsea after eight years at the club, and English striker Bethany England.
"I changed the shape so that we went 1 v 1 everywhere, and made it a little bit more difficult for them. Ji So-yun, she's had a calf injury - trust me, she's put her body on the line for the team, and she was the difference."
Chelsea's gripping turnaround pipped Arsenal's effort to claim their first title since 2019 and denied United's hope of a first spot in the Champions League.
Manchester City eventually claimed the third Champions League place with a 4-0 victory at Reading.
Chelsea becomes the first team in WSL history to win three successive titles, with manager Hayes marking her 10th major trophy for the club.