Mike Baldry's departure from the company in March is described as amicable and he said he plans to stay within the automotive industry.
He joined the BMW Group to head the start-up of its fleet finance and management business in the UK in 1996 and said the time has now come for new challenges.
Since Alphabet's launch in 1997, Baldry has been responsible for the business and led it to become one of the UK's top 15 contract hire and leasing companies and a major player in the growing market for employee car ownership (ECOP) schemes.
Baldry said: 'Being able to play a key role in building Alphabet has been immensely enjoyable and rewarding but I believe that the time has come for me to move on to a new and different challenge. The decision has been made easier knowing that Alphabet is poised for continued successful growth.'
Baldry's last official day as chief operating officer is March 31, when Frank Munk, chief executive officer of BMW Financial Services, takes over.