It accident happened just after 5:30 p.m. at 18th and Chestnut Streets.
Police say Merlin was crossing eastbound on Chestnut Street when she was hit by a vehicle.
The driver 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.