Hayes' side went into the last day of the season ahead of second-placed Manchester City on goal difference in a thrilling finish to the title race.
Chelsea raced into a four-goal lead by half-time at Old Trafford and, with City unable to match that spree in their 2-1 win at Aston Villa, the silverware was destined for Stamford Bridge once again.
Level on points with City, the Blues finished on top with a +53 goal difference compared to City's +46.
It was a fitting finale for Hayes, who was saying farewell to the club she has brought so much success to during 12 years in charge.
Hayes is leaving to take over as United States Women's boss after winning seven Super League titles, five FA Cups and two League Cups.
Her legacy is not just the silverware in Chelsea's trophy cabinet, but also her key role in helping the huge growth in popularity of the women's game in Britain.
City had hoped to spoil Hayes' leaving party by winning the title for the first time since 2016.
But Chelsea had spent 153 days on top of the table, compared to 64 for City, and they rounded off their seventh title in eight seasons in blistering fashion.
City's fate was out of their hands after a 2-1 loss to Arsenal in their previous game, with Chelsea overturning their goal difference deficit with an 8-0 rout of Bristol City.
Chelsea made a dream start against Women's FA Cup winners United in the second minute as Guro Reiten's cross picked out Mayra Ramirez and she rose highest to head home.
Ramirez was a surprise starter from Chelsea on her return from injury, underlining Hayes' knack for making the right call when the stakes are highest.
Six minutes later, Hayes was punching the air in delight as Johnanna Kaneryd clipped a composed finish past United keeper Mary Earps.
Over at Villa Park, Mary Fowler gave City hope with a powerful strike in the 21st minute.
But Ramirez ensured Chelsea would not have to worry about being caught by City.
Surging away down the right flank, Ramirez's pin-point cross was tapped in by Sjoeke Nusken in the 43rd minute.
And Ramirez waltzed through the hapless United defence and smashed a thunderous drive into the roof of the net in first-half stoppage-time.
Hayes' team showed no mercy and Melanie Leupolz scored their fifth in the 47th minute with a close-range effort.
Their title hopes gone, City conceded a 67th minute equaliser to Rachel Daly before Lauren Hemp at least ensured the visitors finished off a winning not.
Fran Kirby, making her final Chelsea appearance after nine years, capped her own farewell with the sixth goal in the 85th minute as the title celebrations started in earnest.
