The victim, Kiwuko Hoffman was struck by a bicycle as she crossed the street.
The police commissioner's SUV was headed southbound on Fifth Avenue when he and a detective came across the woman, unconscious from the crash.
Kelly and a detective, a member of his security detail, jumped from the vehicle and used the injured woman's scarf to stop her bleeding.
She was rushed to New York Presbyterian Hospital-Cornell, where she was listed in stable condition with a fractured skull.
The bicyclist, 24-year-old Jose Segarra Pacheco, was treated for bumps and bruises. He was issued three summonses for improper breaks, no reflectors and no lights on his bicycle.
The incident delayed Kelly's arrival at St. Patrick's Day Mass at St. Patrick's Cathedral, for which he got good natured ribbing from Archbishop Timothy Dolan.
Earlier this month, Kelly came to the aid of a pedestrian who twisted her ankle in a pothole on Broadway in Lower Manhattan.