Jo Lyall, managing director of Mindshare UK, is leaving after more than 20 years to lead a media start-up.
She is moving to become general manager of the UK operation of what she called "an innovative and purpose-driven media brand".
Lyall said she could not disclose her new employer’s name at this stage, but it is "a content and media start-up" that is opening "its first UK office following success in other markets".
She will be "tasked with introducing and growing its innovative media, marketing and consultancy services in the UK market".
Lyall, who has been managing director at Mindshare since 2017 and is a former head of digital at the WPP agency, has got to know start-ups since she set up Grow@Mindshare, an initiative to support fast-growth, digital-first brands.
She has been at Mindshare since 1998 – soon after it was founded in November 1997, when WPP merged the media departments of Ogilvy and J Walter Thompson to form a standalone agency in what was a pioneering move at the time.
Lyall said: "It has been a privilege to grow with Mindshare throughout my career, eventually leading the agency I believe in so strongly and which has returned the favour by believing in me.
"In the last two decades, the media industry has changed almost beyond recognition, but Mindshare’s ethos that the future of media has the audience at its heart has never wavered.
"It’s this core value that I’ll take with me as I embark on the next stage of my journey."
Helen McRae, UK chief executive and chair of Mindshare, western Europe, said: "Jo has had an amazing career at Mindshare, which started in the early days of digital and saw her launch Invention, our first social team, lead planning, create Grow@Mindshare and ultimately become managing director.
"It’s always sad to lose talented people, but the great thing about Mindshare is that not only can you do fantastic work and develop yourself, but you can also discover new areas of interest that take you on a different path."
Lyall developed a side career as a nutritionist during her time at the agency and she has also been keen to fight for greater equality in the workplace.
"For some time now, the solution to gender gaps at a senior levels has been asking for compromise – usually from women," she warned in an article for Campaign to coincide with International Women’s Day earlier this month.
Her departure comes as Mindshare shakes up the UK agency.
Jem Lloyd-Williams, UK chief executive of Vizeum, is joining to become UK chief executive and McRae is expected to focus on international operations.
Mindshare’s clients include Unilever, Nike and Marks & Spencer, but it lost the HSBC and American Express global accounts last year.