He was born on August 22, 1945, in Brooklyn and grew up in Elmont on Long Island. His alma mater is Adelphi University. He and his wife Carol reside in New York City, and have two adult daughters and three grandchildren.
Under Sanford I. Weill, Willumstad helped to steer the 1998 merger of Travelers Group and Citicorp. From 2000 to 2003, he was the chairman and CEO of Citigroup's Global Consumer Group. He was named President of Citigroup in 2002, and assumed the post of Chief Operating Officer in 2003, while continuing to serve as CEO and President of Citibank North America. Some suggested that he had been passed over for CEO in favour of Charles Prince, despite being in charge of Citi's largest business.
Willumstad served as the chairman of the board of directors of AIG from 2006 until 2008, after Maurice R. Greenberg was forced to resign.
Willumstad became CEO on June 15, 2008, after Martin J. Sullivan was ousted. One of Willumstad's initiatives was to repair the rift with Greenberg. In September 2008, Edward M. Liddy succeeded  Willumstad to oversee the company's government-financed asset sale. Willumstad refused a $22 million severance payment for his three months' work at AIG since he was never able to enact his strategy. AIG's stock price declined 97% during his short tenure due to preexisting company conditions.
On October 7, 2008, Willumstad testified before the United States House Committee on Oversight and Government Reform on Capitol Hill regarding the causes and effects of the bailout of AIG.
- "Robert B. Willumstad, President and COO of Citigroup, Inc., to Receive Honorary Degree from Adelphi University at 109th Commencement, May 15, 2005" (Press release). Adelphi University. 2005-04-26. Archived from the original on December 5, 2008. Retrieved 2008-09-16.
- Elstein, Aaron (2003-07-21). "Article: Uncertainty for runner-up; History says passed-over Willumstad will... | AccessMyLibrary - Promoting library advocacy". AccessMyLibrary. Retrieved 2010-05-14.
- "Citigroup No. 2 exec Willumstad to leave". Usatoday.Com. 2005-07-14. Retrieved 2010-05-14.
- "AIG's new CEO Robert Willumstad gets his big break in a tough climate". International Herald Tribune. Associated Press. 2008-06-16. Retrieved 2008-09-16.
- "MarketWatch.com". MarketWatch.com. 2010-05-10. Retrieved 2010-05-14.
- Bawden, Tom (2008-06-17). "New AIG chief Robert Willumstad to look at all the options". The Times. London. Retrieved 2008-09-16.
- Bresnick Kendler, Peggy (2008-06-17). "Edward Liddy Named CEO of AIG". Insurance and Technology. Retrieved 2008-09-16.
- "AIG Gets $85 Billion Fed Loan, Cedes Control to Avert Collapse". Bloomberg.com. 2008-09-21. Retrieved 2008-09-21.
-  Archived September 23, 2008, at the Wayback Machine
- AIG’s ex-CEO Willumstad to forego [sic] $22 million severance, Muckety via Wall Street Journal, 2008-09-22
- Committee Holds Hearing on the Causes and Effects of the AIG Bailout. Retrieved 2008-10-07. Archived October 11, 2008, at the Wayback Machine
- Testimony of Robert Willumstad Archived 2008-10-14 at the Wayback Machine (PDF) Retrieved 2008-10-07