The incumbent, Omnicom's Zimmerman, had handled the fast-food brand's creative and media efforts since 2009.
Resource, a digital agency headquartered in Columbus, Ohio, acquired New York creative shop Ammirati earlier this year.
The two offices will lead integrated brand strategy, advertising, retail activation, website development and all mobile, digital and social media efforts on the White Castle account, collaborating with independent media firm Crossmedia.
"They were thought-provoking," said Kim Bartley, VP-marketing and menu development, White Castle. "We're not making a decision so much as asking them to keep provoking us."
As a 93-year-old brand, one of White Castle's challenges now is remaining relevant. The agency "challenged us to look at the ways of reaching the consumer that we hadn't thought about ... and how to excite more consumers who may not have tried us," she said.
Resource/Ammirati CEO Kelly Mooney said mobile would play a significant role in the agency's efforts to create new interactions with younger, millennial consumers. "Creating new value by integrating mobility is going to be a huge part of the experience and so will having more fun with the content," she said. Other elements will include non-traditional media and events. "We're thinking a little more non-traditional, a little more of a ground swell," said Ms. Mooney.
White Castle spent more than $9 million in measured media in 2013, according to Kantar Media, but the company has yet to finalize its budget for 2015. New creative work is not likely to roll out before the second quarter.