It happened around 11 a.m. near the 1400 block of South 50th Street.
"He wasn't moving or speaking he was just lying there and I am like, 'Oh my god,'" said witness, Andrea Whitmore from Southwest Philadelphia.
"The nurses came out and they tried to assist. They said he was bleeding out and everybody was in a frenzy just standing around because we were like this is really happening in front of our eyes," Whitmore said.
Police say the mail carrier was hit along Woodland Avenue and 50th Street by a man driving this stolen Toyota SUV.
Investigators say the stolen vehicle hit the mail truck, which then hit another parked car. Drakkar Duncan was inside with his cousin. The two only had minor injuries. Duncan says he saw the suspect run away.
"I didn't know if he was running from the crash or something else. I looked behind me and looked to the right and saw the guy on the ground and I was like oh, 'I got to chase that guy,'" Duncan said.
On Thursday, police announced the arrest of 22-year-old Nasir Mitchell in connection with the accident.
He was arrested and charged with aggravated assault, flight to avoid prosecution, aggravated assault by vehicle, an accident involving death or personal injury and other related offenses.
Officers have not released the mail carrier's name but say he's a 46-year-old man.
He's listed in critical but stable condition with a fracture to his right leg.