Thorne is set to step down this autumn, with Fishburn Hedges Group CEO Simon Matthews stepping in to take on her day-to-day responsibilities.
Thorne, who was made CEO in 2011 after five years as MD, has not made her plans public.
She said: ‘I have had 16 fantastic years at Fishburn Hedges but have felt for a while that I need a fresh challenge for the next stage of my career. Over recent months my thinking has crystallised and I have decided to head off in the autumn and will take a little time before I announce my plans. Fishburn Hedges has an exciting future ahead of it, and I wish it well with the next stage in its development.’
Thorne started her career in PR at Communications Solutions in 1991, before heading to The Communication Group three years later.
She joined Fishburn Hedges as a consultant in 1997, going on to rise through the ranks to the top UK role.
Her promotion to CEO came amid a wider shake-up that saw senior departures at Seventy Seven, a part of Fishburn Hedges Group.
Matthews added: ‘Fiona has been a great asset to the business for a decade and a half and will be sorely missed. She goes with the warmest wishes of everyone here and we look forward to hearing what she does next. In the meantime, we’ll be working with the management teams across our group to develop plans for an era when communications disciplines are coming ever closer together.’