Toni & Guy - which has a franchise of almost 400 stores worldwide, including 229 in the UK - has yet to decide if she will be replaced.
Evans was the most senior media relations specialist at Toni & Guy, overseeing a five-strong PR department for three years. She worked closely with a separate 20-strong team responsible for comms tools such as Toni & Guy's in-store magazine and TV station.
She reported to group creative director Sacha Mascolo-Tarbuck and head of media James Tarbuck, who is to take on Evans's responsibilities for an interim period.
Evans's new agency, which has yet to be named, will specialise in advising beauty, hair, fashion and lifestyle clients.
Before Toni & Guy, Evans was managing director of HEPR, with clients including L'Oreal, Lacoste shoes and Vidal Sassoon.
Toni & Guy's London-based PR team advises on comms and branding internationally for all franchises of Toni & Guy and sister stores Essensuals.