He was hired to lead the London office in October 2010 when he was European comms director at eBay.
John Saunders, FH’s regional president for EMEA, said the firm had been working on plans to restructure the managing director role in London and the organisation that would "set it up for the next stage of growth".
"This restructure presented Richard with the opportunity to consider other roles within our organisation. After a transformative four years at the helm of FleishmanHillard in London he has decided, however, that this is a natural point to leave and pursue his next career step," said Saunders.
In the interim, the London office will be run by the remainder of the management team, reporting to Saunders, but the firm said it would be shortly "making further announcements regarding exciting plans for 2015".
Globally, FH is estimated to generate fee income exceeding $550m. In PRWeek’s Top 150 UK PR Consultancies report, the UK operation was estimated to generate £17m in fees.
There is a sense in the industry that the UK arm has underperformed the wider global network over the past decade.
Saunders added: "Richard has brought great insight and passion, both to his role in London and as a member of the wider leadership team within the EMEA region. Richard is enormously talented and he leaves us both with our heartfelt thanks for his great efforts and our warmest best wishes for the future."
Kanareck told PRWeek: "I've achieved what I set out to do in my four years at FH – the business, clients and culture of the London office have been transformed. It was lovely to see the progress recognised with the recent PRWeek win [International Agency of the Year] – that marked a very natural moment to look to the next challenge.
"I’m privileged to have enjoyed senior client-side, agency and management roles within some first-class organisations, and am taking a well-earned break before finalising my next move.
"Fleishman's in great shape and will continue to develop and grow in London. It’s a talented team at all levels, and I wish them all the best as they embark on the next stage."