She joins from Abbott Mead Vickers BBDO, where she was a board strategy director, and will make the switch after Easter.
At Brothers and Sisters, she will report to Phil Teer, the agency’s chief strategy officer, who was previously its strategy director.
Teer said: "Ila brings immense strategic firepower and creative instinct, and we can’t wait to get started."
De Mello Kamath has spent the past eight years at AMV, where she ran the Martini and Bombay Sapphire accounts. She has won two IPA Effectiveness Awards for "who killed deon?" for the Metropolitan Police, including a gold in Best New Learning.
Before AMV, she worked at The Good Agency as its head of planning and MCBD as a senior planner. Before that, she was a graduate trainee at Saatchi & Saatchi.
De Mello Kamath said: "Brothers and Sisters is an extremely innovative, forward-looking agency, perfectly poised to create the kind of brilliant ideas needed in today’s rollercoaster environment, and I am thrilled to be able to join their ride."