Mr Hopkin, whose group entered the AM100 last November, has launched an expansion programme and is poised to take a key role in the Renault/Nissan retail reorganisation.
He takes advantage of Fiat's plans to split the M25 market area into four regions after an expensive six-year experiment with Pendragon which did not work out.
The move will reduce the turnover of the UK's biggest group by around £125m. The change deals a severe blow to Pendragon chief executive Trevor Finn's strategy of increasing profits by building large sales territories and cutting central costs.
Despite several attempts at internal restructuring, the Fiat group only ever made a small contribution to Pendragon operating profits.
Fiat London will now become four geographic sectors with a new partner representing the brand in each one. Glyn Hopkin has bought three dealerships in North-east London. TFL Group has taken over the South-west and the two other sectors are still under negotiation.
After expanding rapidly during 1997 and 1998 Pendragon was forced to retreat last year, closing four sites and cutting senior management by moving administrative functions to company headquarters outside Nottingham.
A Fiat spokesman said the split with Pendragon was “amicable”, denied the experiment had been a failure and said the restructuring was brought about by the end of a seven-year agreement.
Mr Hopkin said Fiat offered his company the territories. The North-east M25 dealerships - formerly Montroe in Woodford, Olympian in Romford and a Libra commercial vehicles site in Forest Gate - will all be renamed Glyn Hopkin.