Gilligan, 55, "became seriously ill on a flight home to New York this morning and passed away," Chenault wrote in the letter, distributed to workers today. A spokeswoman for American Express told ABC News that Gilligan was flying on a corporate jet and the company does not know the cause of the death.
Gilligan was named president of the company in April 2013 after serving as vice chairman since 2007. He joined the financial services company 35 years ago as an intern, Chenault said.
"This is deeply painful and frankly unimaginable for all of us who had the great fortune to work with Ed, and benefit from his insights, leadership and enthusiasm. Our thoughts and prayers go out to his wife, Lisa, and their four children – Katie, Meaghan, Kevin and Shane," Chenault wrote. "He was a proud husband and father, and his love for his family was evident in all that he did."
Gilligan was considered by many to be the heir apparent to Chenault, 63, who has been CEO and chairman of American Express since 2001. Chenault wrote that he asked Gilligan's direct leaders to report to him.