She is the daughter of the late Henry Grunwald, former editor-in-chief of Time magazine and the sister of the author Lisa Grunwald. She grew up on the Upper East Side of Manhattan and on Martha's Vineyard. She attended the Nightingale-Bamford School in New York, and then graduated magna cum laude from Harvard University.
Upon graduation she worked at the Sawyer-Miller Group in New York. Gaining prominence through her work on the successful 1992 Bill Clinton presidential campaign, in which she was director of advertising, Grunwald made television appearances defending Bill Clinton, and helped produce The Man From Hope, the biographical film that was the centerpiece of the 1992 Democratic National Convention.
Mandy Grunwald subsequently founded and became president of Grunwald Communications in Washington, D.C. Throughout the 1980s and 1990s, she served as media consultant for three successful Senate campaigns for Daniel Patrick Moynihan (presumably 1982, 1988, and 1994), as well as for the Ruth Messinger's losing effort in the 1997 New York City mayoral election.
In 1999 she served as a broker across the frosty relations between the retiring Moynihan and Hillary Clinton. The latter would later successfully gain the former's seat in the 2000 New York senatorial election. In 2004, Grunwald worked for the senatorial campaign of Ken Salazar in Colorado and gubernatorial effort of John Lynch in New Hampshire, both of whom won. She also worked for Gifford Miller's Democratic primary campaign in New York City's 2005 mayoral race, who ultimately lost.
The Washington Post identified her as one of the key members of "Hillaryland", Hillary Rodham Clinton's closest advisors beginning with her First Lady days. Grunwald's position as a White House advisor reportedly faded after 1995, when she and others were supplanted by Dick Morris according to the New York Times. However, in addition to acting as an important intermediary between the then Senator Daniel Patrick Moynihan and Hillary Clinton for the 2000 New York senatorial election, Grunwald was appointed the head of campaign media relations for the 2008 Hillary Rodham Clinton presidential campaign.
Described by The New York Times as “smart, tough, gruff, intensely loyal to her candidates, with an air of superiority and great certainty”, she is believed to have been the inspiration for the character "Daisy Green" in the roman à clef of the 1992 presidential campaign, Primary Colors, published by "Anonymous" in 1996. Moreover, before Joe Klein was revealed to be the creator, Mandy Grunwald and her novelist sister Lisa Grunwald were among the authorial suspects. In the 1998 film Primary Colors, the character based on her was portrayed by Maura Tierney. She is also partially the inspiration for The West Wing character Mandy Hampton.
- Elisabeth Bumiller (1999-07-20). "Public Lives: A Top Adviser to a Much-Advised First Lady". The New York Times. Retrieved 2007-10-24. "Ms. Grunwald, 41,..."
- "WEDDINGS; Mandy Grunwald, Matthew Cooper". The New York Times. 1997-11-30. Retrieved 2007-10-23.
- Mark Leibovich (2007-10-26). "A No-Nonsense Style That Was Honed as Advocate and First Lady". The New York Times.
- Gerth, Jeff; Don Van Natta, Jr. (2007). Her Way: The Hopes and Ambitions of Hillary Rodham Clinton. New York: Little, Brown and Company. ISBN 0-316-01742-6., pp. 208-209.
- Greg Sargent (2004-11-22). "The Clinton Factor: Can Bill and Hillary’s former media guru help Gifford Miller beat Bloomberg?". New York. Retrieved 2007-10-24.
- Romano, Lois (2007-06-21). "Gatekeepers of Hillaryland". The Washington Post. Retrieved 2007-07-16.
- Todd S. Purdum (1996-02-01). "The Author Could Not Be Reached for Comment". The New York Times. Retrieved 2007-10-24.
- Rollins, John E.; O'Connor (2003). The West Wing: The American Presidency as Television Drama. Syracuse, New York: Syracuse University Press. p. 52. ISBN 0-8156-3031-X.
- Argetsinger, Amy; Roberts, Roxanne (2008-04-30). "Straight From the Heartthrob". The Washington Post. Retrieved 2010-05-12.