The Dutchman has led the Red Devils to Carabao Cup glory and next month's FA Cup final during a promising first season that looked certain to end in a top-four finish until a recent wobble.
But Ten Hag has righted the ship and United secured a return to European football's top table with a game to spare as Casemiro, Anthony Martial, Bruno Fernandes and Marcus Rashford scored against toothless Chelsea.
This was a club record 16th defeat in a 38-match Premier League season for Frank Lampard's Blues, who saw a bad early Mykhailo Mudryk miss punished by a sixth minute Casemiro header.
Further Chelsea chances went begging across the evening, including the miss by Conor Gallagher in first half stoppage time that was followed by Martial scoring.
Bruno Fernandes' penalty and substitute Marcus Rashford's goal – his 30th of the season in all competitions – wrapped up a victory that was only dampened by Antony's exit on a stretcher and a late Joao Felix consolation.
Ten Hag stuck with the side that beat Bournemouth and took an early lead when Christian Eriksen sent over a free-kick from the left and Casemiro rose high to power a header past statuesque Kepa Arrizabalaga.
The Brazil international punched the air after the goal was given the green light by the VAR after checking for offside.
Things could have been so different had Mudryk – one of Chelsea's three changes– displayed a modicum of composure two minutes earlier.
With Aaron Wan-Bissaka drawn inside and Antony failing to track Lewis Hall's run, the winger somehow turned wide after the promising left-back put it on a plate for him.
Play swung from end-to-end and United had a glorious chance to double their advantage when Eriksen put three team-mates through against one defender.
Fernandes' smart run opened a lane for Antony to play in Martial, but the dithering forward was caught by Cesar Azpilicueta.
It proved Antony's last impact of note as soon the winger left the field on a stretcher having pulled up in agony during an innocuous-looking coming together.
Rashford replaced him and when play resumed Chelsea were again ruing another wasted chance, with Kai Havertz heading a Hall cross wide.
Fernandes saw penalty claims against Wesley Fofana overlooked and Gallagher bent wide as a helter-skelter half headed towards a conclusion.
The Chelsea midfielder would go closer still in stoppage time. Put behind by Enzo Fernandez's fine pass, Gallagher dragged across the face of goal.
It proved another costly miss as Casemiro played an exceptional pass through to Jadon Sancho, who delivered a low ball for Martial to turn in at the back post.
That gut punch was nearly followed by another blow with moments of the second half getting under way.
United, who replaced Luke Shaw with Tyrell Malacia, won the ball through Victor Lindelof and Sancho teed up Fernandes to crash a strike off the woodwork.
Like the first half, this was an entertaining back and forth.
De Gea stretched to tip a deflected Mudryk shot behind before the slightest deflection meant Eriksen could not tap in a low Malacia cross. Kepa got back to claw off the line and Casemiro curled the follow-up just wide.
Hall was denied by De Gea, as was Mudryk's deflected follow-up, and Chelsea saw penalty claims ignored as they looked to reduce the deficit.
But hope of a comeback was ended once and for all in the 73rd minute.
Fofana caught Fernandes with a lazy recovery challenge after being nutmegged by the Portuguese, who stepped up to coolly to convert in front of the Stretford End.
Rashford bent wide as United pushed for a fourth, which he would get in the 79th minute.
Fernandes cut out a lax Fofana pass and teed up the returning forward to put into an empty net after Kepa's first save only slowed the ball.
Alejandro Garnacho hit the bar and was denied by Kepa either side of a fine Chelsea consolation, with Felix running through the centre and ending a fine solo run with a thumping drive.