Goals from Marco Fabian and Anthony Fontana allowed the Union to shake off a brief run of bad form. Philadelphia was only 1-2-2 in its previous five games, including a 4-0 loss at Real Salt Lake on July 13.
Philadelphia goalkeeper Andre Blake made three saves for his third clean sheet of the season.
The victory ensures that the Union (11-6-6, 39 points) will remain in first place in the East for another week, no matter the result of Sunday's match between second-place D.C. United and third-place Atlanta United.
The Fire (5-10-8, 23 points) were playing their sixth game in a busy stretch of eight games in 30 days. Perhaps owing both to fatigue and the humid conditions, Chicago was lacking in offensive sharpness, and spent much of the game on the back foot after surrendering an early score.
In the 11th minute, the Union deployed some trickery on a free kick. Rather than send a cross toward the net from the left side, the ball was instead passed to the outer edges of the box, where Fabian rushed in and blasted a long-range shot into the net.
Fontana entered the game as a substitute for Fabian in the 61st minute and matched his teammate on the scoreboard just four minutes later.
Kai Wagner fought to keep possession deep in the Fire's box, and managed to deflect a pass off a Chicago player that fell right at Fontana's feet. The Union academy product converted the opportunity for his first league goal of the season.
Ilsinho and Warren Creavalle both saw action late in the second half off of Philadelphia's bench. Ilsinho had missed three games with an adductor strain, while Creavalle missed six games with a broken foot.
Chicago captain Dax McCarty missed his first league game of the season. McCarty was sidelined by a minor injury in the Fire's 2-2 draw with Columbus on Wednesday.
The Fire are 0-17-6 in their last 23 league games on the road.
After receiving a yellow card in the 19th minute, Wagner will miss Philadelphia's next game (in Montreal on July 27) due to a caution-accumulation suspension.
Union make it look easy vs. Chicago
Philadelphia strolled past a overmatched Chicago side, winning easily 2-0 at Talen Energy Stadium.
Fabian's incredible goal puts Union in front
Major League Soccer: Marco Fabian (11') Philadelphia Union 1-0 Chicago Fire