"They thought nothing of arming themselves with a gun and ultimately taking a young man's life, leaving four young children without a father.
"I want anyone who was in central Croydon that night who saw this group or may have images of them on their cameras or phones to come forward."
Mr Ellis, from Brixton Hill, was found with a gunshot wound to the head in a vehicle in Duppas Hill Road at its junction with Warrington Road.
He was pronounced dead at St George's Hospital the following day.
The Met said the reward would be given for information that leads to the identification, arrest and prosecution of the persons responsible for the murder.
Detectives specifically appealed for anyone who witnessed the car chase, which involved a dark estate car and a silver hatchback - believed to have been driven by the group of nine - and a dark hatchback in which Mr Ellis was a passenger.
A number of people have been been arrested in connection with the murder. Four men were released without further action while 10 are on bail as police continue investigations.