And it was Mochrie who created the first goal after only three minutes. He made a good forward run down the right channel which was not picked up and he then put over an excellent cross for Darren Watson who slotted home from about six yards.
Aberdeen hit back with a couple of corners and a Jack Milne shot was deflected just wide.
Kevin Hanratty then got involved in the game, he produced a good run which was just stopped by the last defender and then a minute later had a shot which keeper Ruaridh Adams grabbed at full stretch. The home side then equalised on 18 minutes. Michael Dangana showed good strength to hold the ball up and he won his side a free kick 25 yards out from a central position. Ryan Duncan stepped forward to take it and the winger curled the ball past Adams into the top corner. A brilliant strike.
Three minutes later Aberdeen very nearly took the lead but Dangana was very unlucky to see his powerful drive crash off the post. A couple of minutes after that Mochrie forced Ritchie into a smart stop.
Despite the heavy rain both teams were playing some excellent football, moving the ball at speed with the play going from end to end. The pace of Mochrie and Fotheringham was causing the Dons defence some problems and at the other end the impressive Ramsay was getting forward and getting into some good areas. Jack MacIver then tried his luck from the edge of the area on a couple of occasions. The visitors appealed for penalty just before the interval, but referee Andy Milne was well positioned to wave play on. Andy will be part of the team officiating at the Scottish Cup final on Saturday.
A very entertaining half with lots of quality on show and a real attacking intent from both sides.
Half time: Aberdeen 1 Dundee United 1
At the start of the second half Aberdeen had a couple of half chances. Ramsay produced a fine run and cross before Hanratty shot just over from 30 yards. MacIver then had a shot blocked.
After that though United slowly started to take control of the game and dominate possession. The Dons defence were standing up well to the pressure but Aberdeen were struggling to get out of their half.
On the hour mark Mochrie managed to get in behind the back line and his cut back found Kai Fotheringham who looked certain to score but Mason Hancock did brilliantly to deflect the ball to safety. Ten minutes later substitute Finlay Malcolm forced Ritchie into a quite brilliant save from point bank range, the Scotland youth international producing superb reflexes. He then produced another top drawer save to tip a shot from Lewis Neilson round the post.
However, on 84 minutes Ritchie was finally beaten as United scored a deserved winner. Kai Fotheringham picked the ball up on the centre circle and played a one two on the edge of the area before firing a great left footed shot into the roof of the net.
After playing so well in the first half, Aberdeen simply never got going in the second period.
The young Dons finish their U18 season on Friday with a trip to Hearts.