Sirdofsky is CEO of Custard Records (James Blunt, Crash Kings) and president of Coldwater Music Publishing/Sony/ATV - and has represented artists and producers including: 4 Non Blondes, Christina Aguilera, Lamb, Vain and Duff McKagan of Guns N' Roses.
Her current producer roster includes Linda Perry and C.P. Dubb, and she co-manages Matt Morris with Kevin Held.
Richard Stumpf, president of IMU said: "I could not be happier in welcoming Katrina to the Imagem family.
"She's one of the most well respected music executives out there and the fact that she chose Imagem as a partner reflects our shared, progressive vision of what it means to be a music publisher today."
In its first move, the new joint venture has signed Australian artist Chet Faker, LA based writer/producer C.P Dubb and singer/songwriter V Bozeman.