She is known for her roles in the film August: Osage County (2013) and the television series Law & Order: Criminal Intent (2006–2009), Masters of Sex (2013–2014), Eyewitness (2016), and Mare of Easttown (2021), the last of which earned her a Primetime Emmy Award.

[2][3] Nicholson's other notable credits include Tully (2000), Ally McBeal (2001–2002), Kinsey (2004), Conviction (2006), Boardwalk Empire (2011–2013), Black Mass (2015), I, Tonya (2017), Togo (2019), The Outsider (2020), Blonde (2022), and Paradise (2025).

Nicholson's domestic drama credits include William Vincent, Staten Island, Brief Interviews with Hideous Men, Tully, Kinsey, and August: Osage County, for which the ensemble cast was nominated for several awards.

In late 2001, Nicholson became one of the main cast members of the hit show Ally McBeal, portraying Jenny Shaw for 13 episodes.

Julianne worked on the short-lived NBC television drama, Conviction and in what is her best known role, as Megan Wheeler, in the sixth, seventh, and eighth seasons of Law & Order: Criminal Intent.

In 2004, she married British actor Jonathan Cake in Italy; they met while playing a couple on an unaired HBO pilot called Marriage.