The move follows the closure of incumbent agency Judy Bennett PR (PRWeek, 8 September).
The brief is headed by account director Ali Bagriyanik, who joined Brower from a senior press officer role at French Connection in September.
Karen Millen is part of the Reykjavik-listed Mosaic Fashions group, which also owns Oasis. Mosaic merged with rival Rubicon Retail, owner of Principles and Warehouse, in September.
There has been speculation that Mosaic is considering a London listing. This week Karen Millen opened its largest European store - a 4,000 sq ft shop in Dublin.
Fifteen-strong Brower Lewis was acquired by PRCo Group - which also owns travel specialist ZFL PRCo - in June. Judy Bennett ceased trading on 30 September. Other former clients include hosiery brand Pretty Polly and Aristoc - now promoted by Brandnation in the UK.
ALSO... Spanish clothing chain Mango has taken on Bryan Morel PR as its retained UK PR agency, after more than two years with Goodley PR.