Police were called at approximately 4.25pm on Saturday, 25 September to reports of a car in collision with a pedestrian at the junction of Mount Grove and the Watford Bypass.
Officers attended along with London Ambulance Service. The man, aged his late 80s, was taken to hospital where remains in a critical condition.
His next of kin have been informed.
The driver of the car stopped at the scene to assist police with enquiries. There have been no arrests.
Officers from the Serious Collision Investigation Unit are investigating the incident.
Anyone that can assist police is asked call the witness appeal line on 0208 246 9820 or dial 101 and quote CAD5574/25Sep.