Crow left the agency last week. He had worked for Synergy for 18 years, joining when it had 18 staff and was known as Karen Earl Sponsorship. He became CEO when it joined Engine Group in 2008. It now has 41 staff.
Crow will now "pursue new ventures", according to a statement from the agency.
There are no immediate plans to recruit a successor, with the business now being led by MD Lisa Parfitt and chief strategy officer Carsten Thode, assisted by director of strategy Tom Gladstone and creative director Josh Robinson. Dominic Curran continues as CEO of Synergy US.
The news follows a restructuring of parent company Engine, announced last month, and comes the week after the announcement of the departure of Adam Raincock, who led Synergy's comms business. Kate McGregor has been promoted to associate director of comms following Raincock's departure.
Synergy's statement said: "Under Tim's leadership, Synergy became established as a leader in sports and entertainment marketing, winning an array of clients and awards, continually innovating its offering, and launching in the US."
Tim Crow said: "It has been an immense privilege to lead Synergy and to have been part of Engine for the last ten years. I will have nothing but great memories of the people and clients I've worked with and the great work that we've created together. I leave safe in the knowledge that Synergy and Engine will go from strength to strength."
He declined to comment further when contacted by PRWeek.
Debbie Klein, CEO of Engine Europe and Asia Pacific, said "We are sad to see Tim leave the Engine family and enormously grateful for all his hard work, as well as the strong leadership team he leaves behind. We wish him well in his next venture."
Major clients of Synergy include Accenture, Beko, BMW, MasterCard, SSE, Aberdeen Standard Investments and Under Armour.
In his PRWeek UK Power Book 2017 profile, Crow named England rugby coach Eddie Jones as the best manager of reputation in the past 12 months.