Police said the 48-year-old worker was standing behind his parked postal vehicle gathering mail when a silver Honda sedan struck him.
Authorities said the driver continued forward and crashed into another parked vehicle.
The crash was captured on surveillance video.
The incident occurred Thursday shortly after 2:30 p.m. at the intersection of Ashburner and Walker streets.
Sources said the worker lost a leg and suffered severe facial injuries. He was transported to Jefferson Torresdale Hospital in critical condition.
Neighbors said the worker was on his usual route, although he was not driving his usual vehicle.
Friends, coworkers and residents described the victim as a familiar and well-liked presence in the neighborhood.
"This guy was amazing. He went out of his way. Like at my store, Tony's Deals up the street here, I would turn around and say hey can you wait for me? He was ready to go home, and he would wait," said Anthony Perez.
Perez also expressed disbelief about the incident.
"How could it happen to someone like that?" he said.
Neighbors said the postal worker knew many people on his route and regularly made an effort to be kind.
"Actually I was crying just now because I met the guy. He used to come to my store everyday and he was the nicest person ever, always smiling," said Alexander Urbina of Holmesburg Auto.
Urbina said the driver involved in the crash had just left his dealership after picking up a friend's car when the collision occurred.
Those who know the worker said they are hoping for his recovery.
"Prayers. And prayers from everyone," Perez said.
Police said the driver remained at the scene and has not been charged.
The Crash Investigation Division is investigating the incident.