Goals from Nicolas Tagliafico, Steven Berghuis, Sebastien Haller, Brian Brobbey and Edson Álvarez ended a season-long threat from PSV Eindhoven with one game to spare.
PSV had previously beaten Ajax in the KNVB Cup final to deny Ten Hag a third double in Amsterdam, but his side quickly fought back in the league.
Ten Hag's final game with the club he joined in 2017 will come against Vitesse on May 15, before he travels to Manchester to take on the daunting task of getting one of football's giants back to the top.
The 52-year-old will replace interim boss Ralf Rangnick in the Old Trafford dugout having signed a three-year deal with the Premier League side in April.
United have recorded their worst ever Premier League campaign under Rangnick and predecessor Ole Gunnar Solskjaer, with Ten Hag seen as the solution to get them back on track.
The Dutchman will join United as a league winner having crafted two sides not only able to take top honours in the Netherlands, but also compete in the Champions League.
Ajax were minutes from the 2019 Champions League final under Ten Hag's stewardship before losing to Tottenham.
The former defender ventured into management in 2012 with Dutch side Go Ahead Eagles, before working under Pep Guardiola with Bayern Munich's reserves, and later Utretch then Ajax.
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