Allen was born in Portland, Oregon, the daughter of Julie and David Allen. She grew up on Bainbridge Island, Washington, as the middle child of three sisters: older Jennifer (Jenny) and younger Lindsay. She attended Wellesley College as a Sociology major and graduated in 1996. She worked with the NYPD as a domestic violence counselor before pursuing acting.
Allen first established her career portraying Laura Kirk-English DuPres on the soap opera All My Children from 2000–02, taking over the role from Lauren Roman who originated the role in 1995–98. After departing AMC, Allen went on to play Susan Delacorte in Mike Newell's Mona Lisa Smile co-starring with Julia Roberts, Kirsten Dunst and Julia Stiles. She later starred in USA Network's hit series, The 4400. She played the role of Lily Tyler. However, her character was written out of the show before its third season. She did reprise the role of Lily Tyler in 2007 in the fourth season of The 4400 for one episode.
In 2006, Allen was a guest star on House M.D. in the episode "All In", as Sarah, the mother of a sick 6-year-old boy. She was also a guest star in the season 2 episode of Criminal Minds "Open Season" as the victim of two serial killers and alongside Cynthia Nixon in the 2007 season premiere of Law & Order: SVU. She starred as Julia Mallory in the first season of the FX drama series Dirt. Her character was a Hollywood heroin addict. She also appeared as a former love interest of Major Owen Hunt on the ABC's Grey's Anatomy in 2009. Other recent projects include the thrillers Hysteria, From Within and The Collective, and Old Dogs with John Travolta and Robin Williams. She has played regular characters in two television series: Katie Nichols in Terriers in 2010 and Hannah Britten in Awake in 2012. 2014 portrayed Meg alongside side Eli Roth, Peter Stormare and Elizabeth Whitmere, in the American horror film Clown. She also portrayed Linda Kessler in the Drama thriller film Nanny Cam.
In May 2016, Allen guest starred in the thirteenth episode of the first season of Criminal Minds: Beyond Borders, titled "Paper Orphans". She played Emily Wagner, the mother of a young girl that was kidnapped in Haiti.