She will work alongside the other managing partner, Mike Sims, who was promoted in August last year. Both will report to the chief executive, Matt Atkinson.
Arnold spent 12 years at Craik Jones, most recently as the board account director on Orange Business Solutions, RBS credit cards and Young Person's Railcard.
At EHS Brann, Arnold will replace Graham Bentham, who moved to RMG Connect in January. In her new role, Arnold will focus on client services and head accounts including Churchill and Homebase, which the agency won in 2004.
Arnold said: "I am thrilled to be joining EHS Brann at such an exciting time in the agency's growth and development. I look forward to helping guide this dynamic team through the challenges to come."
Arnold will work closely with Sims and Atkinson on driving efficiencies and growth at the agency.
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