Allen first established her career portraying Laura Kirk-English DuPres on the soap operaAll My Children from 2000–2002, taking over the role from Lauren Roman, who originated the role in 1995-1998. After departing AMC, Allen went on to play Susan Delacorte in Julia Roberts's Mona Lisa Smile co-starring with 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 and final season of The 4400 for two episodes.
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.