The two consultancies pitched alongside one another against three other agencies.
Their brief covers all aspects of the financial services firm's direct response work on mortgages and will span TV, press, online and direct mail.
Flourish will handle the creative on the account and All Response Media will manage media planning and buying.
Sean Scannell, Chelsea Building Society's marketing communications controller, said: "We were extremely impressed with the overall package these two organisations offered."
He added: "It's an important time for Chelsea Building Society and we believe the direction we were presented with is the right way we should go. However, we will continue to work with our existing agencies on other projects."
Chelsea Building Society's direct marketing mortgage business was previously handled by WWAV Rapp Collins West.
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