In her new position, her role will be to raise the prominence of planning and strategy at the agency. She will work with John Townshend, the creative partner, and Jonathan Stead, Rapier's managing partner.
Meredith will be responsible for developing Rapier's offering as well as leading its strategy and planning team.
In further changes, the agency's commercial director, Sue Payne, has been promoted to chief operating officer, while Paul Alexander has been promoted from business director to become the head of account handling.
Stead said: "Alison, Sue and Paul have all contributed significantly to our success and I am confident we will be able to build on this momentum with this revised structure."
Rapier has also launched a scheme to enable employees other than the management team to take share options in the agency after they have worked there for two years.
Separately, the agency has strengthened its creative department, hiring Ruth Blair as the head of copy. Blair joins from Joshua, where she was a senior writer on accounts including Whiskas and NatWest.
The move to Rapier reunites her with the art director Steve Broadhurst, with whom she teamed up at Joshua.
Blair will report to Rapier's creative director, David Prideaux, and her task will be to improve the quality of writing at the agency.
Prideaux said: "Ruth's hiring underlines our commitment to the craft of copywriting."
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