Komansky grew up in a family of Russian Jewish immigrants and Irish Catholics. He joined the U.S. Coast Guard before attending the University of Miami in 1965. Komansky joined Merrill Lynch in 1968 as a broker and became a regional director in 1981 and an executive vice president in 1990. Komansky also served as a director and chief executive officer of Merrill Lynch from December 1996 to December 2002, and as a director, president and chief operating officer of Merrill Lynch from January 1995 to December 1996.
He is married to wife Phyllis and has two daughters, Jennifer and Elyssa.
Komansky retired as Chairman from Merrill Lynch in April 2003 after spending 35 years at the firm. Upon his announcement of retirement, John Gutfreund, who ran investment bank Salomon Brothers in the 1980s, said, "David Komansky has been one of the major contributors to the financial community's progress throughout the world during his tenure at Merrill Lynch."
Komansky kept a picture of the Bronx tenement building in which he grew up hanging inside his office.
Among many professional affiliations, he serves as a director of BlackRock and as a member of the International Advisory Board of the British American Business Council. Active in many civic and charitable organisations, he serves on the Board of the NewYork-Presbyterian Hospital.
In July 2005, the Phyllis and David Komansky Center for Children's Health was established at New York-Presbyterian Hospital/Weill Cornell Medical College of Cornell University to make medical care more accessible, advanced, and supportive of the needs of young patients and their families. He is also a member of the board of directors of London-based WPP Group, a business conglomerate.