Fish will join the TSA on 5 June. The charity provides support for families of babies born with the genetic condition which affects the brain, eyes, heart, skin and other organs, and gives grants to research organisations. Fish succeeds Dr Jane Spink, who is leaving to become CEO of Genetic Alliance UK.
She will join the charity 18 months after becoming a senior director at Tonic Life. Prior to working at the Huntsworth-owned agency, she was the joint head of Portland's healthcare team.
Before joining Portland in 2013, Fish had been comms director at the National Institute for Clinical Excellence for nearly seven years and held the same role at the NHS Confederation, the independent body which oversees NHS organisations. Other roles have included stints at agencies Fishburn Hedges and Hanover.
Fish said: "I’m looking forward to bringing my passion and experience to the staff team and working with the TSC community to further improve research, care and support for everyone affected by TSC. My focus is to make sure that everything the TSA does supports the community’s vision of providing hope for today and a cure for tomorrow."
The TSA had income of £1.2m and 12 staff in 2015, its most recent accounts filed at the Charity Commission show.
The charity's chair of trustees Martin Short said Fish had "a wealth of experience that is relevant to leading the TSA".
"I am confident that she will take us forward with vision and sensitivity. Louise has a passion for communities like ours and I am sure there will be exciting developments ahead as she builds on Jayne's work," he added.