Bell was born in New York City, the daughter of Robin Bell, owner of the design firm www.robinbelldesign.com in New York City, and Harvey Siegel. Her father is Jewish and her mother is of W.A.S.P. background. She has an older brother named Luke Siegel who owns a sliding wall and door company called Raydoor in New York City. She also has two half-sisters, Courtney and Mackenzie.

Bell attended Westminster School, Connecticut in Simsbury, Connecticut and then attended Skidmore College in Saratoga Springs, New York before transferring to Rose Bruford College in London, England. There, she landed roles in several theatrical productions, including The Seagull and The Pentecost. She remained in London for a year after graduating, then relocated to Los Angeles, California.

Bell landed her first big job with the lead female role in NBC's War Stories, also starring Jeff Goldblum She made two guest appearances on ER in 2002, then went on to play Alicia Silverstone's wisecracking best friend, Victoria, in NBC's short-lived series Miss Match. In 2004, Bell made her debut as Sally Heep in The Practice, and her character was carried over into the series offshoot, Boston Legal, where she was made a regular cast member until she left the series in 2005. Bell went on to play the lead role in the sci-fi show Surface, which premiered September 19, 2005, and was cancelled in May 2006.

In the fall of 2006, Bell returned to Boston Legal for two episodes, reprising her role as Sally Heep as opposing counsel to Alan Shore (portrayed by James Spader). Her film work includes Chris Fisher's The Hillside Strangler (2004), Love Your Work (2003), Speakeasy (2002), and Slammed (2001). She recently wrapped on her latest film, Pride and Glory (2008), which also stars Edward Norton and Colin Farrell. She also has a leading role in the upcoming thriller Still Waters (2008), starring opposite Jason Clarke and Clifton Collins, Jr. She co-starred with Paul Ruddand Eva Longoria Parker in the movie Over Her Dead Body, which opened on February 1, 2008, and also appeared in the 2008 film What Happens in Vegas.