Gabriel Farfan and Kyle Nakazawa also scored for the Union, whose six goals marked a franchise-worst for Toronto.
Maicon Santos scored both goals for Toronto (2-5-6), who pulled to within 3-2 with an early second-half flurry before letting the game get away, drawing boos from many of the 20,122 fans at BMO Field.
"That was a bad, bad performance by us," Toronto coach Aron Winter said.
Philadelphia (6-3-2), in only its second season in Major League Soccer, thoroughly dominated fifth-year Toronto in the first half, scoring its first goal in just the second minute of action. Prowling the right side of the box, Farfan gathered in a cross from the left sideline by Jordan Harvey and put his right-footed shot into the bottom left corner past goalkeeper Stefan Frei.
Mapp made it 2-0 in the 11th minute on a left-footed shot from outside the box into the bottom right corner.
Toronto did show some life in the 35th minute when Joao Plata set up Richard Eckersley for a left-footed blast from just outside the area that went just over the bar.
Hopes of a comeback suffered a setback just before the end of the first half on a splendid goal by Nakazawa, a right-footed shot from the center of the box to the bottom left corner with assists by Mwanga and Sebastien Le Toux.
Santos injected some life into Toronto with a goal in the 50th minute, breaking in after taking a long pass from Eckersley. He worked around goalie Faryd Mondragon, who had come well off his line, and fired the ball into the middle of the empty net.
Santos scored again in the 60th minute from close in and in front of the goal to pull Toronto to within a goal. But Philadelphia dashed any hopes two minutes later when Mapp scored his second goal of the game on a left-footed shot from outside the box into the upper left corner.
Philadelphia, which last won on the road March 19 at Houston, scored again in the 72nd minute when Mwanga converted a cross from Le Toux, then in the 89th with an easy goal by Mwanga.
Notes: Toronto made two changes at the start of the second half with Julian de Guzman and Dan Gargan replacing by Javier Martina and Danleigh Borman, respectively. ... Toronto was without injured forward Alan Gordon (groin) and midfielder Jacob Peterson (hamstring). ... Toronto's next game is against Sporting KC at BMO Field on June 4. ... Philadelphia and Toronto split their two games last season, each team winning at home. ... Toronto was looking for its first win since May 7, a 2-0 decision over Houston. Its other MLS win this season was against expansion Portland 2-0 on March 26.(Copyright 2011 Philadelphia Union. Reprinted by permission)