The departure is effective December 31.
Gallagher, an Ohio native who also served as president of Cliffs' North American business unit since 2007, took over the seat in January 2011.
During his tenure, he oversaw commercial and logistics responsibility for all of Cliffs' global businesses, and also has negotiated and executed contracts and rail and vessel transportation deals for the producer.
Cliffs also said that Gallagher, who initially began his career with the company in 1981, had held various key leadership positions throughout his service and was instrumental in Cliffs' transition from being primarily a North American mine management company to an international diversified mining and natural resources group.
“Don is an outstanding and accomplished leader who has led the organization through some of the most challenging times in the industry and has been instrumental in Cliffs' growth and profitability over many years," board chairman James Kirsch said.
“We are sincerely grateful to Don for his dedication and commitment to Cliffs over the past 32 years — he personifies the resiliency of Cliffs.”