Barcelona flourished early on as new signing Ronaldinho, who turned down a move to the English champions, stroked the ball around with typical Brazilian flair and imagination despite being booed by the United faithful before the start.
United manager Alex Ferguson rested strikers Ruud van Nistelrooy and Ole Gunnar Solskjaer. Also missing from the line-up was Argentina midfielder Juan Sebastian Veron, a transfer target for Chelsea.
Without those three, United appeared lacking in ideas for the opening exchanges.
Ronaldinho's running reaped dividends in the ninth minute when he lofted a perfectly weighted cross for Dutch striker Patrick Kluivert to head past Tim Howard, United's new American goalkeeper.
But, despite further Barcelona pressure, United levelled after 25 minutes. Winger Ryan Giggs found space on the left and crossed for an unmarked Diego Forlan to beat keeper Victor Valdes with a diving header.
United, spurred on by skipper Roy Keane, went ahead 11 minutes later when Nicky Butt's centre from the right took a deflection and man of the match Forlan pounced to score his second goal.
Barcelona came out for the second half without Ronaldinho and Kluivert. United made two changes after 52 minutes, bringing on David Bellion for Darren Fletcher and van Nistelrooy for Uruguayan Forlan.
BRILLIANT GOAL
Van Nistelrooy produced a goal of individual brilliance after 85
United made it four wins out of four on the tour. They had also beaten Celtic 4-1, Mexico's Club America 3-1 and Juventus 4-1.
Ferguson told reporters he was delighted with the trip. "Everything has been great and we're looking forward to the start of the season," he said.
"It took us some time to get going really," he said of Sunday's game. "But by the end of the (first) half I thought we were in control and I thought we should have scored more goals."
The Scotsman said he chose not to play Veron so that all of his midfielders could play two full games on the tour.
Ferguson singled out Howard, recently signed from the New York/New Jersey MetroStars, as the player who benefited most from the tour.
He said his second-half stop from Xavi was "one of the best saves you'll see".
"Tonight he was confident, relaxed and he enjoyed it," Ferguson said of Howard, adding that he "will definitely compete for the No. 1 jersey".
Sunday's game was the first to be played at Philadelphia's new Lincoln Financial Field, home of the National Football League's Philadelphia Eagles, and drew a sell-out crowd of 68,396.