Wheatley had been a non-executive director at the AIM-listed PLC since the firm floated in 2000, and became chairman four years ago.
He stepped down with immediate effect this week and has resigned as a non-executive director at the £10.8m group.
Chief executive Peter Gunning said: "The board wishes to thank Les for his years of service, his leadership and the professionalism he has brought to the board. They would also like to wish him well in his future professional and personal endeavours."
“We won’t be doing an external hire, and therefore are shrinking the size of the board. We will decide the new chairman at a future board meeting,” he said.
Grafenia has three non-execs: Toronto-based Pavel Begun, Hamburg-based Jan Mohr, and London-based Conrad Bona.
Gunning and finance director Alan Roberts are its two executive directors.
Grafenia posted a loss of £260,000 in its most recent results, which reflected a period that saw it sell off its Dutch business and the departure of founder Tony Rafferty.
As well as working to grow its existing offerings organically, the firm aims to expand through acquisition in order to create economies of scale to justify the expense of being a PLC.
