Siff started appearing in television series in 2004. She appeared as an
Alcoholics Anonymous speaker during an episode of
Rescue Me in Season 2. She also had roles on
Law & Order: Special Victims Unit, ''
Grey's Anatomy, and Law & Order''. She played
Rachel Menken Katz on the series
Mad Men from 2007 to 2008, which earned her a nomination, along with the rest of the cast, for a
Screen Actors Guild Award for Outstanding Performance by an Ensemble in a Drama Series. She also appeared in a small role as a burn victim on
Nip/Tuck during that time, before being cast as
Dr. Tara Knowles on
Sons of Anarchy in 2008. She has appeared in such films as
Then She Found Me (2007) as Lily,
Push as a
psychic surgeon (called a Stitch) named Teresa Stowe, sent to help Nick (played by
Chris Evans),
Funny People (2009) as Rachel,
Leaves of Grass (2010) as Rabbi Renannah Zimmerman, and
Concussion (2013) as Sam Bennet. She appears in the 2016 Showtime series
Billions. She starred in an independent indie film called
A Woman, A Part (2016) as well as
One Percent More Humid (2017). In 2017, she narrated the audio book ''
Gwendy's Button Box by Stephen King and Richard Chizmar and the sequel Gwendy's Magic Feather'' by Chizmar. Starting in 2018, she has been serving as the television spokesperson for
Betterment, an online investment service. In 2020, Siff provided the voice for the titular subject of "
Polly Platt: The Invisible Woman", over the course of a season of
Karina Longworth’s film history podcast
You Must Remember This. In 2026, Siff made her West End debut playing
Joy Davidman in
Shadowlands at the
Aldwych Theatre. ==Personal life==