The agency won the account following a pitch against Anomaly and Creature. The process was handled by Ingenuity and kicked off in June.
The process was triggered by the arrival of Cadbury’s marketing director, Jeff Swan, as the new marketing director at KP Snacks.
Swan said: "WCRS demonstrated an impressive combination of strategic rigour and creative firepower – we look forward to working with them to build the McCoy's brand and business."
McCoy's spent £1.4 million on media in 2014.
Last year, the McCoys brand used Crispin Porter & Bogusky, to create an ad called "McCoy’s ultimate".
BMB handles the KP Snacks advertising excluding McCoy’s, but was asked to create the latest McCoy's campaign while the crisp brand held a pitch to find a new creative agency.
On Tuesday, the brand launched 'Go Bold', a campaign that urges men to cast off traditional notions of masculinity.
A film shows a plumber belting out Ella Henderson’s Ghost in his car. When the protagonist gets to a petrol station to buy some McCoy’s, he breaks into song again, adding a few dance moves for good measure.
The film will be backed by print ads with messages such as "Go bold. Go enjoy that candlelit bath", which encourage men to eschew masculine stereotypes.
Matt Edwards, the chief executive at WCRS, said: "McCoy's is a brand with huge untapped potential both creatively and commercially and we're thrilled to be joining the team."