Mack, who raised at least $200,000 for U.S. President George. W. Bush's reelection, credits Clinton as willing to work with Republicans and having a firm grasp of financial industry and healthcare issues, BusinessWeek reported in its May 7 edition.
BusinessWeek also said the heads of Lehman Brothers, Goldman Sachs and JPMorgan Chase have all favored Democrats in their political giving patterns over the past few years, citing contribution tracker PoliticalMoneyLine.
A representative from Morgan Stanley confirmed that Mack and his wife Christy Mack have made a personal decision back Clinton.
People familiar with Mack's support for Clinton told CNBC that the chief executive has worked closely with Clinton as a senator and believes she understands the importance of globalization. He also cites her understanding of the financial services industry and focus on healthcare as reasons for his support.
Mack is chairman of New York Presbyterian Hospital. His wife founded the Bravewell Collaborative Foundation, a healthcare philanthropic organization. John and Christy also recently helped Duke University open its Center for Integrative Medicine.