She is known for playing Alexandra Cabot in the NBC crime drama series Law & Order: Special Victims Unit (2000–2018) and its spinoff media.

March has appeared in the films Head of State (2003), Mr. & Mrs. Smith (2005), Falling for Grace (2006), The Treatment (2006), Jesse Stone: Night Passage (2006), Confessions of a Shopaholic (2009), and Origin (2023).

[6] At Northwestern University, March played Helena in A Midsummer Night's Dream in Chicago, where she continued to pursue her stage career.

March is best known for her role as Assistant District Attorney Alexandra Cabot on the NBC crime legal drama series Law & Order: Special Victims Unit, who she played as a main character from 2000 to 2003, and reprised in recurring appearances on the show until 2018.

The show, which saw March in a leading role and was part of the Law & Order universe, was cancelled after one 13-episode season following a negative reception from critics and audiences.

"[15] In 2003, March made her film debut with a supporting role in the political comedy Head of State opposite Chris Rock.

March also appeared in the 2009 film The Invention of Lying as the woman Ricky Gervais's character tells the world will end unless she has sex with him.

She was cast as a regular character on the short-lived CBS legal comedy-drama Made in Jersey in 2012, but she left the series after the pilot episode.

[22] On March 9, 2019, The Social Ones premiered at the Hammer Theatre Center in San Jose, California, as part of the 2019 Cinequest Film and Creativity Festival.

[23][24] Laura Kosann wrote and directed the film, which also features Richard Kind, Jackie Hoffman,[25] Debra Jo Rupp, and Peter Scolari.

[17] She subsequently appeared in the Lifetime biographical television film A House on Fire, where she played Debora Green, a former doctor and convicted murderer.

[43][44] She currently serves on the boards of the Panzi Foundation,[45] OneKid OneWorld,[46] the Whitney Museum of American Art,[47] and The Gotham Film & Media Institute.

[2] March wrote an essay, published in June 2016, that detailed her experience with breast augmentation surgery in 2014 and the subsequent removal of the implants due to infection.

[54] She was introduced to Benton, the founder of Andor Capital, by a mutual friend in October 2015, several months after her divorce from celebrity chef Bobby Flay.