Mitchell was formerly managing director at Glass Page.
Her remit will include building on relationships with existing contacts and growing business at the agency, whose clients include Bentley, Bombay Sapphire, Danone Baby Nutrition, Lloyds Pharmacy and Tuffnells.
Mitchell joins leadership and management event specialists’ founding partners Richard Bridge and Nick Terry on the board.
Top Banana commercial managing director Nick Terry said: "Susan has a deep-seated understanding of how live events can help organisations deliver against key strategic objectives and she understands the importance of leadership communication. She will be a real asset to the company."