Mack joined Weber in 2011. His previous roles include director of strategy at Freuds and senior planner at Porter Novelli.
In a statement, Weber Shandwick London MD Rachel Friend said: "After almost six years, Adam Mack is leaving Weber Shandwick to return to the world of strategic consultancy.
"It’s true what everyone says, Adam really is one of the nicest people you could meet and work with. He has done a tremendous job to grow our strategic planning and analytics function both here in London and across EMEA. We thank Adam him for all his work and wish him the very best for his future."
Other recent people moves at Weber include the promotion of Pam Jenkins to president of its global public affairs practice, replacing Ranny Cooper, who is stepping down.
Last month, the firm's UK and EMEA CEO Colin Byrne announced his intention to step down next year.
IPG-owned Weber Shandwick, the world’s second-largest PR agency, is listed third in PRWeek UK’s Top 150 Consultancies 2017 table – UK revenue rose an estimated 11 per cent to £47.75m last year.