Confirming the move, an Engine spokesman said: "This follows the recent transaction and careful thought about the future strategy and direction of the group. We thank Peter for all he has done for Engine and wish him well with his future ventures."
Scott was one of the four founders of ad agency WCRS in 1979 and led efforts to build it into a wider marketing services group, which included the rebrand to Engine in 2005, the formation of MHP in 2010 and the acquisition of Mischief in 2011.
Lake Capital, which agreed the sub-£100m deal for Engine in July, is putting it together with its entertainment and content marketing firm Trailer Park and its research arm ORC.
Scott's role at Engine changed from chief executive to vice-chairman and head of global strategy, with Lake Capital co-founder and chairman Terry Graunke coming in above him as executive chairman.
One industry source told PRWeek: "Many saw this as inevitable since Engine were bought by Lake Capital. They wanted change and Peter's role had changed a lot since the acquisition."
The Engine spokesman declined to comment on whether Scott still holds shares in the company but said the company has no plans to appoint someone else to his role.
Engine currently owns 13 marketing services agencies, ranging from digital shop Jam to direct marketing specialist Partners Andrews Aldridge.