French midfielder Sebastien Le Toux scored for Philadelphia in the 23rd minute.
Playing their first official match of the 2010/11 season, at PPL Park, Celtic finished the game with a flourish, peppering the Union goal in the dying moments.
Celtic had one goal disallowed, with another chance, the last shot of the game, bouncing off the crossbar. Union keeper Brian Perk smothered the loose ball as the whistle sounded to end the game.
New signings Charlie Mulgrew and Joe Ledley both started on the bench for Celtic, which are expected to announce the signing of young Mexican defender Efraim Juarez by the end of the week.
South Korean defender Cha Du-Ri, also a new signing, is expected to join the team shortly.
Celtic now travel to Canada for a date with Manchester United, in Toronto, on July 16. The game will be played indoors at the Rogers Centre, home of Major League Baseball's Toronto Blue Jays.
Next is a friendly with another MLS team, the Seattle Sounders, owned by American TV personality Drew Carey, on July 18, and then Sporting Lisbon, at Fenway Park, home of the Boston Red Sox, on July 21.
Celtic's Scottish Premier League season starts on August 14, with the identity of their first round Champions League qualifying opponent to be revealed Friday.
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