The London firm will serve as MFI's press office, as well as carrying out CSR work, issues support and brand building for the retail giant. Siren has been briefed to increase the chain's share of voice in the consumer press.
This strategy comes after a rocky few years for the furniture chain. Parent company Galiform sold MFI for a nominal £1 fee in 2006 to concentrate on its other operations.
Last year, a new private equity-led management team invested £125m in the business.
MFI spent the first quarter of 2008 searching for a new agency, handing Siren the account to support a five-year turnaround plan.
MFI marketing director Michael Greenhalgh said the furniture chain was now 'partway through its turnaround strategy'.
He added: 'It is now time to have like-minded business partners on board who can deliver PR results to match.'