McCormack was born in Plainfield, New Jersey to Norah, a clinical therapist, and William McCormack, who owned a car dealership and an ice cream parlor; the two divorced in 1990. She has a sister, Bridget, and a brother, William. McCormack graduated from The Wardlaw-Hartridge School in Edison, New Jersey in 1987 and went on to attend Trinity College in Hartford, Connecticut, majoring in English and painting.

McCormack began her acting career at age 12 performing in Gian Carlo Menotti's Christmas opera, Amahl and the Night Visitors. She continued her stage work in many more musicals, and finished an acting program at the William Espor Studio. She has worked in many New York theater productions, including The Atlantic Theater Company, Alice's 4th Floor and Naked Angels, where she appeared in Jon Robin Baitz's A Fair Country.

McCormack's key television roles include Justine Appleton on Murder One (1995–1997); and Kate Harper on The West Wing (2004–2006). She also starred in the HBO series K Street, which only aired for ten episodes.