Sukhjimder Singh was arrested moments after the crash in the middle of town around 5:30 p.m.
The 8-year-old girl was run down while riding her pink-and-blue bicycle. She suffered serious injuries but is expected to survive, being listed in stable condition. Her injuries include a broken leg, a broken collar bone along with head and facial injuries.
Police say Singh was speeding when he was stopped, and officers allegedly found plastic bags with heroin residue under the car seat.
Singh is facing a number of charges, including aggravated assault.
"This is gutless, this is cowardly, this stinks!" said Upper Darby Police Superintendent Michael Chitwood. "This offends me."
He continued, "To leave a little 8-year-old baby on the highway and not render assistance? It's probably because he was high on drugs. He's in jail and he won't be getting out of jail any time soon."
Singh is being held at the Delaware County prison.