Ole Gunnar Solskjaer was unable to sustain the upturn in form he inspired at United after arriving in December, instead slumping to sixth, and his former side added to the Old Trafford outfit's woes.
Nathaniel Mendez-Laing was fortunate to be awarded the 23rd-minute penalty that he dispatched, but United could have no complaints with the same man's second goal as Cardiff departed for the Championship with their heads held high.
Mason Greenwood became the hosts' youngest starter in the Premier League at 17 and threatened throughout, yet there was no way past the brilliant Neil Etheridge.
Greenwood provided United's chief threat from the outset and his header drew a stop from the scrambling Etheridge, before Marcus Rashford dragged wide.
But Cardiff's first notable forward foray resulted in the opener as referee Jon Moss controversially adjudged Diogo Dalot to have fouled Mendez-Laing, who sent David de Gea the wrong way from the spot.
A determined response saw Etheridge help a deflected Greenwood drive onto the post and then save from Jesse Lingard, while the impactful debutant also prodded wide.
However, a slow start to the second half and a questionable tactical call cost United. Scott McTominay was moved into defence and dived into a challenge, leaving space for Josh Murphy to run through and square for Mendez-Laing to tap into the net.
Etheridge made unorthodox stops from Rashford and substitute Anthony Martial to preserve a two-goal advantage that Neil Warnock's men ultimately deserved, with the home fans still remaining inside an increasingly empty Old Trafford left to reflect on a season they will want to quickly forget.
What does it mean? Solskjaer's strong start long forgotten
A run of six matches without victory in all competitions is not how any team wants to end the season, let alone Manchester United. Cardiff went down this season and were still able to cut through the hosts at will. United have gone backwards in recent months and major surgery is required in the transfer market.
Greenwood v Etheridge
One entertaining battle saw players on either side impress. Greenwood certainly provided some cause for optimism for United, with his movement simply too much for the Cardiff defence. But the visitors had Etheridge, one of the stars of their season, on song again and he excelled against the teenager early on.
Cardiff enjoy Ole's demise
"You're getting sacked in the morning," sang the Cardiff fans, who might claim to know a thing or two about Solskjaer's tactical limitations. His side played well for periods of the first half, so the decision to shift the entire team around when Phil Jones went off at the break was a strange one. Makeshift defender McTominay was directly at fault for the second goal.
Key Opta Facts
- 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 City became the first newly promoted team to win away at Mancester United in the top flight by a margin of two or more goals since Nottingham Forest in December 1977 (4-0).
- Cardiff won an away league match against Manchester United at Old Trafford for the first time since April 1954, ending a run of seven games without a win (W0 D2 L5).
- United 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).
United have an awful lot of work to do in a potentially pivotal transfer window if they are to improve on this season's disappointing finish. Signings will be expected, but doubt surrounds the futures of various star players, including Paul Pogba and De Gea.
Cardiff are preparing for life back in the Championship and will hope to have Warnock, now 70 years old, leading their charge again.