The Reds beat Santa Clara Sporting Club 5-2 on Monday afternoon local time, with goals from Jesse Lingard (2), Larnell Cole, Will Keane and Tom Thorpe.
Santa Clara were late additions to the 12-team tournament, after original entrants CS Cartagines from Costa Rica were unable to participate, and the Californians quickly made an impact by taking the lead through Alfredo Moran Perez after five minutes. United's response was also swift however, with Larnell Cole levelling in the 8th minute.
The American side went back in front on the half-hour mark, this time through Daniel Moran Perez, only for Keane to notch the Reds' second equaliser shortly before the interval.
Lingard's first goal in the 51st minute, created by Keane, put United in front for the first time in the match but Warren Joyce's side had to wait until the closing stages of the game to seal the victory with a quick-fire double. Lingard and then Tom Thorpe struck in the 80th and 81st minutes respectively.
The third and final game in Bracket (Group) A for Warren Joyce's side is against the Mexican side Tigres at 20:30 local time (six hours behind the UK) on Wednesday evening. Meanwhile in Bracket C, the other English representatives Everton have won one and lost one game so far.