Dembele's fourth goal of preseason opened the scoring at a sold-out Red Bull Arena just before the break. The French forward linked up well with Raphinha before driving home.
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Barca missed a string of chances to add to their tally, with Robert Lewandowski forced to wait for his first goal for his new club once again, before Memphis finally added the second late in the second half after Daniel Edelman had been sent off for the hosts.
The Red Bulls actually had the first shot of the game, Dru Yearwood's volley dipping just over, but they wouldn't muster a shot on target across 90 minutes of Barca domination.
Lewandowski, a 45 million summer signing from Bayern Munich, was making his third start for Barca but he could not find his first goal for the Blaugrana, despite having a string of chances here.
Dembele, who scored twice in the 2-2 draw with Juventus in Dallas earlier in the week, drifted in from the left, played a one-two with Raphinha and then drilled a low shot past Coronel.
Xavi made five changes at the break, but Lewandowski stayed on and the chances kept coming. His deflected effort was tipped over the bar by goalkeeper Ryan Meara, on at half-time, before another save brought a wry smile from Lewandowski, aware that this might not be his night.
He was eventually substituted after 72 minutes but Barca remained in control. Pierre-Emerick Aubameyang missed the target after a quick free-kick and Ansu Fati bent wide after a great run by Alejandro Balde.
Meara then made another fine stop after his teammate Hassan Ndam turned an Aubameyang cross towards his own goal.
Nealis expected Meara to come for a long ball over the top but instead the Dutch forward nipped in to seal a 2-0 win for Barca, who end their U.S trip with three victories and one draw.