According to the Philadelphia Medical Examiner's Office, Johnston died from blunt impact trauma and her manner of death has been ruled accidental.
Johnston, 26, of Southampton, went missing on July 10 after a visit to Philadelphia.
On Sunday, police discovered her body near the Byberry Creek off Townsend Road in Philadelphia.
Family members reported Johnston missing after she never returned home from a friend's house located at 12th and Vine streets in Philadelphia.
Police say Johnston was last seen driving a silver 2016 Ford Focus with Pennsylvania registration.
SEE ALSO:Family, co-worker of fatal hit-and-run victim Adriana Moreno Sanchez speak out
A break in the case came Saturday when investigators found Johnston's car in a wooded area off Woodhaven Road, near Roosevelt Boulevard in Northeast Philadelphia.
Johnston's family had hired private investigator Kevin Ryan, who was tracing what would have been her most likely path home. Ryan had flown a chopper and discovered Johnston's Ford Focus in a wooded area near a ramp off Roosevelt Boulevard and Woodhaven Road.
It's still unclear how the car ended up in the woods. Lower Southampton Township Police Chief Ted Krimmelsays it's possible Johnston had trouble traveling around a curve.
"Went airborne, up and over the guardrail. She crashed into a tree, probably about 30 feet in the air," Krimmel said. "We believe she was ejected from the car through the sunroof. And the place where the car landed...it was 49 yards from the roadway to where her car landed. You couldn't see it from the street or the air."
Authorities believe recent flooding in the area may have moved the body into the nearby creek. Investigators also believe recent tornadoes helped clear tree debris that made it impossible to spot the vehicle during early searches.
SEE ALSO:Father, 2 daughters and niece killed in NJ crash