Throssell (pictured) joined the UK arm of the WPP network agency from Iris Worldwide in 2016. It's unknown what her next steps will be.
Ogilvy UK chief marketing officer Nina Jasinski said the agency has no immediate plans to replace her and her responsibilities are being divided amongst the existing team.
"Bree has played an important part in delivering the Ogilvy communication message, she’s been great fun to work with and she takes with her all our very best wishes for her next role," said Jasinski.
Throssell said: "Ogilvy has been on a path of transformation over the last few years; one that has welcomed both remarkable industry leaders and clients - all of whom I feel enormously grateful to have worked alongside. The global transformation of the agency has been quite the chapter to be a part of and provided a dream landscape for someone working in the world of communications. It feels the right time now to explore what else is out there."
The agency had its global ‘refounding day’ on 5 June, which saw the formal relaunch of Ogilvy as a single entity with a unified P&L. The formal Ogilvy brands, including Ogilvy & Mather and Ogilvy PR, were phased out, with the specialisms incorporated into the one agency.