The former Barnsley CEO moved to the City Ground in 2021 and helped oversee their return to the Premier League last season.
Forest confirmed he will leave by mutual consent to pursue other opportunities.
"Never have I been a proponent of, nor in fact, have I ever been any good at goodbyes. It is much easier to give thanks and recognize those who made my time at Forest so special," he said in a personal statement posted on the club's website.
"Nottingham is a community of people who put the work in before the talk. Who pour themselves into what matters most and commit to the genuine causes that allow them to progress.
"The unbridled passion for this football club, passed down through generations, reverberates throughout the sport. That passion is the true north that guides the players, the staff and all at the City Ground.
"I would be remiss to not acknowledge those who helped make my time at the club successful. Thank you, first and foremost, to Evangelos Marinakis who allowed me to realize a dream I did not know I had.
"To Socrates Kominakis, Miltos Marinakis and chairman Nicholas Randall KC, thank you for believing in my stewardship.
"Finally, thank you to all the players, staff, and co- workers who over the last 18 months helped build the club to where it now stands. Everyone should take great pride in the achievement.
"My gratitude for the welcome received, and the treatment of my wife and I by this community cannot be bound by words."