Jamiro Monteiro set up the late first-half goal by Przybylko as the Union stretched their unbeaten streak to four matches (3-0-1). The goal was Przybylko's second of the season and 25th in 56 matches with the club.
Philadelphia goalkeeper Andre Blake made seven saves while recording his fourth shutout of the season and 42nd of his career.
D.C. United lost for the fifth time in their past six matches. They held a 17-7 edge in shots.
The Union broke the scoreless tie in the first minute of first-half stoppage time.
Przybylko was on the line of players when Monteiro spotted him and sent a pass ahead. Przybylko timed his move perfectly and no D.C. United defenders went with him as he collected the pass and sent a short left-footed shot past D.C. United goalkeeper Jon Kempin (one save).
The play was initially ruled as offside but video review displayed Przybylko was onside when the pass was made and the goal was awarded.
D.C. United had a superb chance to knot the score in the 63rd minute when Kevin Paredes sent a blistering right-footed shot on goal. Blake thwarted the opportunity by using his hand to deflect the ball over the crossbar.
Felipe had a solid chance in the 90th minute for D.C. United but his header also was wide to the right.
D.C. United's best chance in the first half came in the 24th minute when Paul Arriola took a free kick and sent a hard grounder toward the left side of the net. Blake made the save with his right hand and teammate Jakob Glesnes cleared the ball out of trouble.
Philadelphia had an opportunity in the 43rd minute. Monteiro took a close-range, right-footed shot and the ball caromed off the chest of Brendan Daniel Hines-Ike and toward Kempin, who made the point-blank save.