Best will step down as DDB London's chief people and strategic officer, but will continue to work with the agency as a consultant one day a week.
Best began his career at BMP in 1975 as a trainee planner under Stanley Pollitt and within six years joined the board as planning director.
After the merger of BMP with DDB in 1989, Best served as the chairman of BMP DDB through the 90s. He joined the DDB Worldwide board in 1991 and served as the president of DDB Northern Europe, Australasia and the Middle East and Africa.
He was the chairman of the UK Advertising Association from 1997 to 2002.
The retirement announcement comes before Stephen Woodford joins DDB London as its chief executive at the end of January and coincides with the exit of the managing director, Jorian Murray.
"Events like Ken Kaess' death (the president and chief executive of DDB Worldwide in New York) last year do make you think about things," Best told Campaign. "But I've been lucky to work with BMP and DDB and couldn't wish to do much more than what has already been done."