Danny Welbeck, Chris Smalling and Fabio Da Silva all scored first-half goals while substitutes Adnan Januzaj and Jesse Lingard hit the back of the net in the second period.
Lam Ka Wai's stunning long-range strike and a solo effort from Alex Akande punished some slack United defending to reduce the deficit.
But despite showing some defensive frailties David Moyes would have been pleased with some fine individual performances on the night by the likes of Wilfried Zaha, Januzaj and Lingard.
In front of a 40,000 strong crowd and a poor pitch, the men in red fired three goals in the space of 10 minutes.
England striker Welbeck tucked home the first after Ashley Young saw his effort crash off the post.
Even with one-time Fulham defender Zesh Rehman in their line-up, Kitchee were no match for United in the air and defender Smalling climbed highest to direct Tom Cleverley's fine cross into the far corner.
The home side were unable to cope with the tricky Zaha and it was his pass which created the third of the night.
Fabio collected the ball before calmly sliding it into through the legs of Rehman and into the far corner with the outside of his right boot.
Within four minutes of the restart, Buttner had created a goal for Januzaj who finished with a precise header.
A capacity crowd were on their feet to acclaim Lam when he gave United goalkeeper Ben Amos no chance with a thunderous shot into the top corner.
And after United had failed to take a succession of opportunities, Alkande pulled another one back midway through the half thanks to a fine solo strike.
Lingard put the glossing on a better attacking display with a superb curling finish from the edge of the area as he finished top scorer on tour with four goals.
Dutchman Robin van Persie was introduced for the final few minutes. He also had the honour of wearing the captain's armband.
United wind up their pre-season campaign against AIK Fotboll in Stockholm on August 6 before starting their Premier League season at Swansea City on August 17.