Chief Executive Jack Schuessler stepped in to take the helm, with what he said will be a "fanatical" approach to righting the ship that has delivered negative same-store sales results in nine out of the last 12 months.
Wendy's (WEN: news, chart, profile) shares dropped 2.2 percent, or $1.05, to $46.27 Monday. Since Aug.1, Wendy's shares have dived 14 percent.
Mueller, 53, had been with the company since 1998 after a 24-year stint at privately held Burger King, according to Marketwatch. He has served as president for five years, most of which were marked by new menu choices and innovations. Wendy's, for example, was the first of the hamburger chains to successfully sell premium salads.