Fish is set to join Tonic on 4 January as senior director, public affairs and corporate communications.
Fish joined Portland in March 2013. She previously worked as director of communications and engagement at the NHS Confederation and director of communications at the National Institute for Health and Clinical Excellence (NICE).
In her new role, Fish is to work from the London office of Huntsworth Health, which includes Tonic Life in its portfolio.
Mary Smiddy, global CEO of Tonic Life Communications, said: "Louise is a leading figure in the industry and brings a remarkable degree of insight, experience and skills to the team. In particular, she will advise clients, new and current, on issues and crisis management, managing the media, and building effective relationships with government, policy makers and other influencers."
Fish said: "I look forward to putting my experience into practice to provide public affairs and corporate communications support to their enviable list of current clients, and to build a new offering at this award-winning agency."
Brearley joined Portland in May 2014 from Publicis Life Brands Resolute and was made partner in January 2015. She has been joint head of Portland Health since April and will now assume sole responsibility for the 14-strong practice.
Portland said the health practice has grown by over 30 per cent this year.
Oliver Pauley, Portland's UK managing director, said: "After a year of incredible growth and exceptional new business wins, I am delighted that Jane will lead the health team from January 2016. It’s a very exciting time for the practice and we have an ambitious plan for next year, including significant investment in new specialisms such as medical education."
He added: "When Louise and I first discussed developing a health practice at Portland in 2012, I don’t think either of us could have imagined how successful it has become. I am very grateful for her commitment to the business and wish her very well for her future plans."