Mitchell, who starts this month, joins Ruby's three founding partners: Richard Megson, Matthew Davis and David Miller. He will bolster the agency's planning capabilities and be responsible for strategy across Ruby's clients.
Like the founding partners, Mitchell will have equity in the business, with The Red Brick Road remaining the majority shareholder.
Mitchell joins Ruby after two years at glue, where he worked on strategy for Sky, T-Mobile, Eurostar, News International and COI. Before that, he worked at Harrison Troughton Wunderman.
Miller said: "We're a hungry young agency and Ben's strategic and creative skills are just what we're looking for to strengthen our offering."
The Red Brick Road launched Ruby as a sister agency to offer clients direct marketing, digital, tracking and audience segmentation. Megson and Davis were formerly a creative team at CHI & Partners, while Miller was a board director at Proximity.