Barry Nightingale joined the group, which owns the Frankie & Benny's and Garfunkel’s restaurants, last June after a one-year stint as chief financial officer at Monarch Airlines where he helped the airline return to its first profit in three years.
His departure comes almost a year after his predessor, Stephen Critoph, left. Critoph’s exit was announced at the same time the group issued a profit warning due to weak trading at its restaurants.
In early 2016 the company also lost chairman, Alan Jackson, who was replaced by Debbie Hewitt.
The group said it has started the search for Nightgale's successor and would update the market in due course.
Chairman Hewitt said: "On behalf of the board, I would like to thank Barry for the valuable contribution that he has made over the past year, as we began the company's turnaround process, which continues to progress well."
During Nightingale’s time at the Restaurant Group, he faced a difficult task in turning around the business after a string of poor trading updates.
Last month the company reported an 11% drop in full year pre-tax profits to £77mln and a 3.9% decline in like-for-like sales as it restructured the business and revamped its restaurant menus.
The group said it experienced a "challenging" trading year across its leisure brands - including Coast to Coast, Frankie & Benny's and Chiquito.