The shooting happened around 8:50 p.m. Monday in the 3400 block of G Street in the Kensington section.
The video shows the man, identified as 28-year-old Joel Johnson, walking between several cars stopped in traffic.
Johnson stops at the detective's vehicle, and a few moments later shots are fired and Johnson falls to the ground.
He was taken to the hospital in critical condition and is expected to survive.
Family members have said Johnson has special needs, often begs for change, and doesn't carry a weapon.
Police say the detective involved in the incident thought the man was a carjacker and armed with a gun, which prompted him to fire his weapon from inside his unmarked car.
Philadelphia Police Commissioner says, however, there was no weapon.
"It's not automatically an issue of whether there's a weapon or not. I know for some people they think that's the way it should be - obviously, it's the first thing we look for. But I'm always going to be very candid with you when I know there's not one. So I'm not going to sit here and tell you that we haven't found one, we don't expect to find one," Ross said.
Ross says Johnson was known to panhandle in the area.
"Why does he deserve this? He stays to himself. He begs for money and just goes home," Johnson's niece told Action News.
Niece of shooting victim says her uncle is mentally challenged and was begging for quarters when the @PhillyPolice officer shot him. According to detectives, the shooting victim approached the officer in an unmarked car. The officer thought he had a gun and shot him. @6abc pic.twitter.com/rzrqfvw0fR— Christie Ileto (@Christie_Ileto) May 21, 2019
The 7-year veteran of the force was on-duty and traveling back to his headquarters at East Detectives, which was not too far away.
The detective was in his unmarked vehicle, in plainclothes and had just been processing a scene.
He was stopped at a red light when Ross says Johnson emerged from between two parked cars with his arms extended.
Police say he was shouting something, it's unclear what, and he approached the rolled up window on the detective's driver side.
"The detective believed what he saw was a firearm in this male's hands. The detective believing he was going to be robbed or something was going to happen to him produced his firearm, discharged his firearm we believe at this time three times," Philadelphia Police Captain Sekou Kinebrew said.
Johnson was struck once in the torso.
His brother said, "A lot of people say he just made a hand gesture for change like he always does. The officer was on his phone and when he looked up he got startled, he got nervous, and started shooting through the window."
The detective has been placed on desk duty, which is protocol, as the investigation continues.