- Her father was a stage actor in New York.
- Began acting professionally in Philadelphia after graduating from nearby Bryn Mawr College, and won a Theatre Alliance of Greater Philadelphia Barrymore Award in 1998 for her supporting role in Henrik Ibsen's Ghosts.
- Made TV debut in a 2004 episode of Third Watch and movie debut in the 2007 legal drama Michael Clayton.
- Had a recurring guest role on the first season of Mad Men as department store heiress Rachel Menken.
- 2007, Screen Actors Guild Awards — Outstanding Performance by an Ensemble in a Drama Series: Nominee
- 2014, Critics' Choice Television Awards — Best Supporting Actress in a Drama Series: Nominee
- Paul Ratliff — Husband
- Bryn Mawr College, Bryn Mawr, Pennsylvania, United States (BA in English, 1996); New York University, Tisch School of the Arts, New York, New York, United States (MFA in Drama, 2004)
- Birth Name: Margaret Siff
- Birth Place: Bronx, New York, United States
- Birthday: June 21, 1974, Gemini
- Profession: Actor
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