Reaser was born in Bloomfield, Michigan to a housewife mother and a lawyer turned restaurateur and substitute teacher father. She is a former student of the Juilliard School. She once appeared in a play in London for only two audience members.

Reaser may be best known for co-starring in the feature films Stay and The Family Stone. In October 2004, Interview magazine hailed her as one of the "14 To Be" emerging creative women. In preparation for her role on Saved, she hung out in an emergency room, researching the behavior of medical staff.

Reaser was nominated in 2007 for an Independent Spirit Award as "Best Female Lead" for her work in the film Sweet Land. In early 2007, she appeared on Grey's Anatomy in a recurring guest role as an unidentified pregnant woman suffering from amnesia. She makes a comeback in season 4, when her character returns to Seattle Grace for Dr. Alex Karev, with whom she shares a strong mutual attraction.