Police believe he was part of a group who were close to a silver Renault Traffic minicab parked outside Dominos Pizza on Alexandra Avenue on Sunday morning at 1am.
The cab drove away, but the driver soon realised the rear door was open and a man was rolling on the road behind him.
The driver returned to the injured man and has not been arrested.
The victim was treated by paramedics at the scene, who described his head-injuries as “life threatening”, before being taken to a central London hospital where he subsequebtly died. A post mortem examination will now be carried out, and his next of kin have been informed.
Police are unsure on why the man was in the cab and are keen to speak to other members of his group, who had left the area when officers arrived.
Detective Sergeant Chris Osborne said: “We know this man was with a group of what appeared to be friends or colleagues moments before he was injured.
“He has now died and I would urge anyone with information to come forward so that we can establish the precise events.”
Anyone who can assist the investigation should call 020 8991 9555. To remain anonymous call Crimestoppers on 0800 555 111.