Ellis, who is a former member of the Campaign Power 100 Next Generation and one of Management Today’s Power Part Time 50, which acknowledges business leaders working flexibly.
She has been on maternity leave and the appointment marks her return to the industry. Ellis replaces Katie Lee who left to join Sunshine last year.
Ellis said: "I’m passionate about nurturing and developing the next generation of talent, which [Gravity Road] has in abundance alongside an incredibly experienced senior team."
The hiring follows the appointment of Seb Royce as executive creative director and Duncan Snowden as director of tech and distribution at the agency.
Ellis has previously worked at brands including Boots and Barclays.
Mark Eaves, co-founder of Gravity Road, said: "Gravity Road’s role in helping marketers transform brand building through more progressive behaviour around content and tech is growing in scale. Sarah brings something unique to this: front-line experience from within big brands."
