Wrexham continued their encouraging pre-season campaign, handing Michael Carrick defeat in his first match as the club's permanent head coach.
Sam Smith, a former Manchester United academy player, scored the decisive goal six minutes before half-time, converting Lewis O'Brien's cross with a simple finish after the ball appeared to slip through Harry Maguire's legs.
The Championship side followed up last week's goalless draw with Wisla Krakow by producing another disciplined display, while United's preparations began with defeat despite fielding a strong first-half line-up in front of 20,000 spectators at the Olympic Stadium.
The match also marked the debut of Brazilian midfielder Andrey Santos following his move from Chelsea earlier this week. The 22-year-old started alongside Mason Mount at the base of midfield and produced an energetic first-half display, recovering from an early shaky moment to make several important interceptions, win a number of duels and show a good range of passing.
Speaking before kick-off, Carrick praised his new signing's early impact at the club.
"He's settled in really well, he's a fantastic character," Carrick said. "He's settled in so quickly and he's buzzing to be here, you can see that."
Wrexham looked the more threatening side for much of the opening period. Bailey Cadamarteri forced Ayden Heaven into a recovery challenge after nicking possession on the edge of the area, while United's first meaningful opportunity came when Joshua Zirkzee fired narrowly wide midway through the half.
United almost gifted themselves the lead in the 36th minute when Smith inadvertently headed a corner onto his own crossbar while attempting to clear. However, the striker quickly made amends at the other end, arriving at the far post to tap home O'Brien's delivery and give Phil Parkinson's side a deserved advantage.
Carrick made sweeping changes at the interval, replacing every outfield player as an academy-based side, captained by Tyler Fletcher, took over for the second half. Aaron James was the only Wrexham player to remain on the pitch after the break.
Although United enjoyed more possession after the restart, they struggled to create clear-cut chances. Radek Vitek prevented Wrexham from extending their lead with an excellent save to deny Davis Keillor-Dunn, while Dan Ward comfortably dealt with United's only efforts on target from Ethan Wheatley and Shea Lacey late in the contest.
A late penalty appeal from Dan Gore was waved away as Wrexham comfortably saw out the victory.
Phil Parkinson's side will now travel to the United States for friendlies against Leeds United, Liverpool and Sunderland before opening their competitive season away to Middlesbrough in the Carabao Cup on August 7. They begin their Championship campaign against Cardiff City on August 17.
Elijah Upson, the son of former Arsenal and England centre-back Matthew Upson, is set to join the Gunners after leaving North London rivals Tottenham, with an official announcement expected in the coming days.
Manchester United remain determined to challenge for the biggest prizes, with director of football Jason Wilcox insisting the club's ambitions have not changed despite an ongoing rebuilding process.
Aston Villa have completed the signing of Switzerland international Johan Manzambi from Freiburg in a club-record deal worth up to 59.5 million pounds, beating Newcastle to one of Europe's most sought-after young midfielders.
Youri Tielemans believes the time was right to join Manchester United, saying the club's ambition and desire to compete for major honours convinced him to make the move.
