She joins from online retailer notonthehighstreet.com, where she was head of communications.
At Portland, Winmill will advise on the corporate comms programmes for several key clients, including Heathrow and Hyatt.
At notonthehighstreet.com, Winmill established the retailer's first corporate communications function.
She has previously held senior roles at Freuds and in-house at L’Oréal. During her career, she has delivered corporate communications programmes for brands including Asda, PepsiCo, Intel, Samsung and LinkedIn.
"Louise combines fabulous in-house experience with an agency perspective," said Portland senior partner and head of corporate Sam Ingleby. "Her expertise will strengthen and broaden our corporate communications offer, helping us to grow our practice, and give our clients the best communications support in the business. We’re thrilled that Louise is joining Portland."
Winmill’s appointment follows several senior strategic hires in recent months, including Steffan Williams, Newgate’s former CEO, and Laura Trott, who worked as a special adviser to David Cameron.
Winmill said: "Very few agencies today can compete with the breadth of Portland’s corporate offer and the calibre and expertise of its people. This, combined with its impressive and ever-growing portfolio of big-name clients, makes it a unique and exciting business that I feel privileged to be joining."
