DeWitt appeared in Showtime's series United States of Tara as Tara's sister, Charmaine. She appeared in Season 1 of the AMC series, Mad Men, as Midge Daniels, lead character Don Draper's bohemian mistress, and returned for one episode in Season 4. She co-starred with Ron Livingston in the 2006–2007 Fox series Standoff. She played FBI hostage negotiator Emily Lehman. DeWitt has also appeared on television in Law & Order: Special Victims Unit, Sex and the City, Rescue Me, and Love Monkey.
DeWitt is featured in the film Margaret, opposite Anna Paquin and Mark Ruffalo. Additional film credits include Purple Violets, Off the Black, Shut Up and Sing, The Great New Wonderful, Blackbird and How I Got Lost. She plays the role of Rachel in the Jonathan Demme-directed movie Rachel Getting Married (2008) alongside Anne Hathaway, for which she won several critics' awards and a Satellite Award for Best Supporting Actress.
She was married to actor Chris Messina from 1995 to 2006. She married actor Ron Livingston, her co-star in Standoff, on November 2, 2009, in San Francisco. In May 2013, they announced that they had adopted a daughter. Gracie James Livingston was born on April 29, 2013. In December 2016, the couple announced they adopted another daughter, Esperanza Mae Livingston, the previous year.
DeWitt played Ben Affleck's character's wife in the corporate drama The Company Men. In 2011 she played Hannah, one of the three lead characters in the critically acclaimed Your Sister's Sister, opposite Emily Blunt and Mark Duplass. In 2011, she played Renee Blair in the film A Little Bit of Heaven starring Kate Hudson.
In 2012, she played Alice in Gus Van Sant's film Promised Land, released on December 28, 2012.
DeWitt performed in numerous off Broadway plays. Most notably, she starred in John Patrick Shanley's Danny and the Deep Blue Sea at the Second Stage Theatre; George S. Kaufman's The Butter and Egg Man at the Atlantic Theater Company; and Craig Lucas' Small Tragedy, for which the entire cast won an Obie Award. From May 4–23, 2010, DeWitt appeared in MCC Theater's Off Broadway play Family Week, written by Beth Henley and directed by Jonathan Demme. In its review of the play, the New York Times stated that DeWitt's lead performance "has many affecting moments as the beleaguered Claire."
In 2016 she appeared in the multi award-winning La La Land as Laura, the sister of Ryan Gosling's character Sebastian.