James S. McDonald, head of investment-management firm Rockefeller & Co. and a board member of NYSE Euronext, died on Sunday in Massachusetts, according to people familiar with the matter. He was 56 years old.

In a statement Monday night, Barclay McFadden III, who identified himself as a friend of Mr. McDonald's family, said he "took his own life." The family has "no further comments beyond this," the statement added. "

Jim...