DeWitt was born in Queens, New York. She is a granddaughter of James J. Braddock. She played the role of neighbor "Sara Wilson" in the movie Cinderella Man, which depicted the life of her grandfather.
Rosemarie lived in Hanover Township, NJ and is a graduate of Whippany Park High School. She performed in several high school productions including the role of Princess Puffer in the "Mystery of Edwin Drood"
She attended New College of Hofstra University where she majored in creative studies and was a member of Alpha Phi. She had additional training at the Actors Center in New York.
DeWitt has a background in theater, consisting of numerous off-Broadway plays. Most notably, she starred in Danny and the Deep Blue Sea at Second Stage; The Butter and Eggman at the Atlantic Theater; and Craig Lucas' Small Tragedy, for which the entire cast won an Obie Award.
DeWitt appeared in the 2007 season of the AMC series, Mad Men, as lead character Don Draper's mistress, Midge.
She 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 upcoming 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 and Blackbird.
She currently resides in Los Angeles.
She is the aunt of electronic musician John DeWitt of Undrig