Kate Howe, Dentsu Aegis Network’s executive director, creative, customer experience and commerce, is leaving the business after three years.
Howe, who is also chair of Gyro UK and director of B2B, international, is joining MSQ Partners as executive director in February.
At MSQ she will report to chief executive Peter Reid and join the board and will be required to help shape and implement the group's growth plans, which last month inlcuded backing Simon Davis who has set up Walk-In Media.
A spokesman for Dentsu Aegis said there "are no immediate plans to replace her" in either of her roles.
Euan Jarvie, Dentsu Aegis Network UK and Ireland chief executive, said: "Kate is an authentic and inspiring leader and has been a key member of my leadership team this year. While we are sad to see Kate leave, she leaves us with a strong management team in place to continue to grow our brands and our people. On behalf of everyone at Dentsu Aegis, I would like to thank her and wish her the very best for her next move."
Dentsu Aegis bought Gyro in July 2016 and incorporated its business-to-business media agency Enterprise into Gyro.
Before Gyro, Howe held posts including group marketing director at Gala Coral.
Reid added: "Kate’s arrival is a critical piece in the MSQ jigsaw. We are developing quickly and having Kate on board will help drive our ambitious plans forward even more rapidly. She’s a highly collaborative and inspirational leader who brings vast experience in helping agencies and networks grow."