Police say it was just after 5:30 p.m. as Merlin was crossing eastbound on Chestnut Street at 18th Street when she was hit by a vehicle.
The driver of the striking vehicle did stop.
Merlin was taken to Hahnemann Medical Center where she remains in critical condition. The extent of her injuries have not been released.
Merlin runs the Commission of Aging Office.
Philadelphia police are investigating the incident.