The appointment follows a pitch involving a number of other RBS roster shops. Peter Deane, the marketing director of Churchill, Green Flag and Privilege, oversaw the process.
WCRS&Co will now be responsible for shaping Churchill's overall social media strategy, with its first work for the brand breaking later this year.
The agency has been the incumbent on Churchill's above-the-line account since 2004, and recently launched a campaign featuring the brand's Churchill dog mascot posing as a shopkeeper in a traditional British general store.
The ad saw clumsy customers entering the shop to enquire about the benefits of Churchill's products.
It used the line: "You can count on Churchill."
Before that, WCRS&Co had created a number of campaigns for Churchill depicting the mascot's encounters with numerous celebrities, such as Rolf Harris, Ricky Hatton and Melanie Sykes.
Last year, Churchill handed its direct marketing business to CHI & Partners.
That appointment, which was made without a pitch, signalled an end to Churchill's longstanding relationship with its previous incumbent, EHS 4D.
CHI already held a place on the RBS Group roster as the incumbent on the Green Flag advertising business. It also handles DM for the Royal Bank of Scotland.