Redundancies will reach double figures during a month-long review, which kicked off in March.
In the interim, Jon Shilling will lead the division, while a new leader is sought.
Hillier joined Grass Roots in 2005 as commercial and measurement director before being promoted to divisional director in January 2012. He is now on gardening leave.
Unwin leaves after almost a year as a group account director for the automotive sector, working with brands such as BMW, Ford, Toyota and Jaguar Land Rover.
Grass Roots plans to consolidate its four divisions as one legal entity. This will not involve the relocation of offices.
The news follows chairman David Evans's departure from the company in February, and the appointment of a spate of WPP directors at the company. Andy Lister was subsequently made Grass Roots' chief operating officer.
David Hackett, deputy chariman of Grass Roots, said: "We are looking to recruit a single leader to run the Marlow branch who will assume sole responsibility to drive the business forward.We have streamlined the business so we can bring together the four strands of the company to become the supplier of choice for the corporate events industry."