Her arrival follows last week's appointment of the former Wieden & Kennedy senior creative David Bell as its creative director.
Sanderson will take over the day-to-day running of the Poulter agency (whose clients include Jeyes cleaning products and the house-builder Redrow), as well as building client relationships.
She replaces Nick Goring, who takes over as the client services director across the group's divisions, which include direct marketing, sales promotion, PR, interactive and brand consultancy.
Mark Buckle, the Poulter group development director, said: "Sarah will bring about a step change for us. We've always had a strong strategic input into our clients' business. But we need to showcase our creative product more.
A lot of ad business can be transferred into other areas."
Sanderson, who spent four years away while raising a family, had been working as a consultant for Boots, where she managed its relationship with JWT.