Match ends, Manchester United 0, Cardiff City 2.
Nathaniel Mendez-Laing scored twice as relegated Cardiff City won on their final Premier League appearance to heap more misery on Manchester United.
Mendez-Laing opened the scoring for Cardiff from the penalty spot after being brought down in the box by Diogo Dalot, before scoring his second after the break.
Goalkeeper Neil Etheridge proved the visitors' hero as he foiled several United chances on a frustrating day for the hosts, who fielded six academy graduates.
Defeat means United have won just one of their final seven games of the season as city rivals Manchester City won the Premier League title for the second season in succession.
Cardiff travelled to Old Trafford just a week after confirmation of their relegation to the Championship and looked set to bid farewell to the top flight with a whimper as United dominated the opening exchanges.
But it was United who finished the match with their tails between their legs, unable to defy a resolute Bluebirds defence as they rained chance after chance on Etheridge's goal.
Cardiff's victory was their first over Manchester United since 3 April, 1954, when manager Neil Warnock was just five years old.
- 'Not a cat in hell's chance' - Warnock says he won't manage in Premier League again
- Manchester United v Cardiff as it happened
Nightmare season finally over for United
With Europa League status secured for next season, this campaign proved one to forget for United as they look to rebuild over the summer.
Ander Herrera confirmed this week he would leave the club after a five-year stay, and the exit door at Old Trafford could be busy with Antonio Valencia also departing.
Solskjaer looked to have turned United's fortunes around as they went 12 league games unbeaten after his arrival.
But the wave they were riding crashed in March, and the Norwegian will spend the summer building his own team rather than the one he inherited from Jose Mourinho in December.
But while he will be keen to bring in players from elsewhere, the club's youngsters will no doubt be much involved next season.
At 17 years and 223 days, Mason Greenwood became the youngest player to start a Premier League game for the Red Devils, and he more than showed his potential against Cardiff as the sole ray of sunshine in a gloomy United performance.
He had a handful of chances to score, twice going close with headers before Mendez-Laing scored his first and testing Etheridge not long after with a deflected effort on goal.
Eighteen-year-old Angel Gomes came off the bench late on with he and Greenwood scuffing chances to pull something back for United as they end the season in sixth, 32 points adrift of champions Manchester City.
Bluebirds end top-flight stay on a high
The 2018-19 season has been one in which Cardiff has united as a club, and manager Warnock was all smiles at full-time despite his side's demotion.
Whether he will still be in charge of the club next season remains to be seen, with owner Vincent Tan keen to retain the 70-year-old who has one more year remaining on his contract.
The Bluebirds can be proud of the performance with which they downed United, weathering an early storm to shock their hosts by taking an early lead.
With nothing at stake, Mendez-Laing coolly slotted home his spot-kick, sending David de Gea the wrong way as his team-mates in defence stopped every United break in its tracks.
It was more of the same after the break as Mendez-Laing, completely unmarked just metres out from the United goal, tapped in a Josh Murphy cross to double his and Cardiff's tally.
But Cardiff had Etheridge to thank for keeping them in the game as the keeper's rapid reactions kept out a Marcus Rashford bullet header just moments after the second goal, before stoppong a powerful Paul Pogba effort with 20 minutes to go.
Cardiff finish the season just three points from safety but with some pride intact.
'It was not good enough' - what they said
Manchester United manager Ole Gunnar Solskjaer: "Today was not the problem. You can have all the possession and chances you want but we concede easy goals and don't score at the other [end]. We have a long and hard way to be where we want to be.
"Mason Greenwood was brilliant, he was our best player by a mile and that says a lot about the kid. He came into a team with a lack of confidence and you can see that has taken a toll. I'm not surprised by his performance, we know he is capable of that.
"It will take some time to close in on the top, we finished five or six points behind third, fourth, fifth and that has to be our aim. The top two teams have set a standard higher than before and we have to take up the next challenge.
"The work starts now. Everyone has the summer off but we have to come back with a different mentality and different attitude of being a Man United team.
"The ones who played, did well but it was not good enough."
Cardiff manager Neil Warnock: "We were super today, so were the amazing fans. I am really proud of the whole club. We did not set out to defend and thought we had a chance to put a dent in some of the bookies today.
"My last game in the Premier League, I wanted the players to play for me today. With a bit of luck, we would have stayed up. We have developed as a team.
"I am having a meeting with the chairman and chief executive on Monday. The club is in a strong position from when I took over and they can push on like Burnley did when they went down. I will leave the club in a good state whenever I leave.
"The fans have been brilliant all the time. I have never worked as hard as I have this season, I have never had so many knock-backs and then I get a performance like that after being relegated.
"I felt the club needed me when the Emiliano Sala tragedy happened. I felt I had to be the front man. It was difficult and I have never experienced anything like that. We came through it intact and I am proud of that."
Man of the match - Neil Etheridge (Cardiff)
- Manchester United have only kept two home clean sheets in the Premier League this season, their fewest at Old Trafford in a top-flight campaign since 1962-63 (2).
- Cardiff became the first newly-promoted team to win away at United in the top-flight by a 2+ goal margin since Nottingham Forest in December 1977 (4-0).
- Cardiff won an away league match against United at Old Trafford for the first time since April 1954, ending a run of seven games without a win (W0 D2 L5).
- United have finished 32 points behind the champions this season - more than they finished behind the top-flight winners when they were relegated in 1973-74 (30 points).
- Cardiff attempted 11 shots in the first half against United - the most by an away team at Old Trafford in the first half of a Premier League game this season.
- Mason Greenwood (17 years, 223 days) became the youngest player to start a Premier League game for United.
- Cardiff's Nathaniel Mendez-Laing is only the second player from a newly-promoted team to score a brace away at Old Trafford in the Premier League after Louis Saha for Fulham in August 2001.
- 1de Gea
- 20DalotSubstituted forA Valenciaat 74'minutes
- 4JonesSubstituted forMartialat 45'minutes
- 15PereiraBooked at 44minsSubstituted forGomesat 74'minutes
- 6PogbaBooked at 61mins
- 14LingardBooked at 71mins
- 25A Valencia
- 4MorrisonBooked at 90mins
- 5Ecuele Manga
- 19Mendez-LaingSubstituted forHoilettat 80'minutes
- 15BacunaBooked at 28mins
- 17GunnarssonSubstituted forRichardsat 58'minutes
- 11MurphyBooked at 61mins
- 14De Cordova-Reid
- 10ZohoreSubstituted forWardat 74'minutes
- 23K Harris
- Jonathan Moss
- Shots on Target
Second Half ends, Manchester United 0, Cardiff City 2.
Attempt saved. Mason Greenwood (Manchester United) right footed shot from the right side of the box is saved in the top right corner. Assisted by Anthony Martial.
Corner, Manchester United. Conceded by Joe Bennett.
Attempt missed. Antonio Valencia (Manchester United) right footed shot from outside the box is high and wide to the right following a set piece situation.
Attempt blocked. Paul Pogba (Manchester United) right footed shot from outside the box is blocked.
Sean Morrison (Cardiff City) is shown the yellow card for a bad foul.
Marcus Rashford (Manchester United) wins a free kick in the attacking half.
Foul by Sean Morrison (Cardiff City).
Attempt missed. Angel Gomes (Manchester United) right footed shot from outside the box is high and wide to the right. Assisted by Mason Greenwood.
Foul by Scott McTominay (Manchester United).
Danny Ward (Cardiff City) wins a free kick on the right wing.
Attempt missed. Jesse Lingard (Manchester United) right footed shot from the right side of the box misses to the left. Assisted by Anthony Martial.
Attempt blocked. Anthony Martial (Manchester United) right footed shot from outside the box is blocked. Assisted by Antonio Valencia.
Substitution, Cardiff City. David Junior Hoilett replaces Nathaniel Mendez-Laing.
Attempt missed. Marcus Rashford (Manchester United) right footed shot from the left side of the box is too high. Assisted by Ashley Young.
Scott McTominay (Manchester United) wins a free kick in the attacking half.
Foul by Josh Murphy (Cardiff City).
Corner, Manchester United. Conceded by Nathaniel Mendez-Laing.
Corner, Manchester United. Conceded by Lee Peltier.