Dennings, the daughter of Ellie Litwack, a poet, was born to a Jewish family in the Bryn Mawr/Wynnewood area of Philadelphia, Pennsylvania, where she was raised on June 13, 1986. She has an older brother, Geoffrey Litwack. Dennings was home schooled. She moved with her family to Los Angeles, California around 2002 so that she could act full time. Her parents initially considered her idea of pursuing an acting career “the worst idea ever,” Dennings told Interview magazine in 2007.
Dennings began her career in 2000 on the HBO cable television series Sex and the City, in the episode “Hot Child in the City”, where she played a 13-year-old girl who hired Samantha to handle publicity for her bat mitzvah. She was then cast as Bob Saget’s daughter on the short-lived TV series, Raising Dad. Since then, she has appeared in a number of television shows and movies, including The 40-Year-Old Virgin (2005), playing Catherine Keener’s daughter, and the Hilary Duff film, Raise Your Voice (2004). Dennings also appeared as the character “Zoe” on several episodes of ER in 2005 and 2006, and was a guest star on CSI:NY, playing Sarah Endecott in the 2005 episode “Manhattan Manhunt”. Dennings appeared in the film Big Momma’s House 2 (2006), starring Martin Lawrence.
In 2008, Dennings starred in Charlie Bartlett, the story of a wealthy teenager, played by Anton Yelchin, who acts as a psychiatrist for his new public high school. Dennings played Susan Gardner, Bartlett’s love interest, and the daughter of the school’s principal, played by Robert Downey, Jr.. Dennings also appeared in The House Bunny, playing Mona, a pierced, misfit sorority girl. Dennings will star with Michael Cera, of Superbad and Juno, in the film Nick and Norah’s Infinite Playlist, a teen romance based on a novel of the same name by Rachel Cohn and David Levithan. Dennings shot the film in New York over 29 consecutive days of night shoots during the 2007–2008 Writers Guild of America strike. Nick and Norah’s Infinite Playlist will be released on October 3, 2008.
Dennings has been blogging since January 2001, and has since also transitioned into video blogging on YouTube. She is a self-proclaimed lover of baby animals, Sho-jo manga, and cheese. She is an avid reader and is friends with author Andrea Seigel. She is also a huge Steve Martin fan. Her brother Geoffrey also writes a blog. In 2005, Dennings dated Ira David Wood IV (brother of Evan Rachel Wood), whom she brought to the world premiere of The 40-Year-Old Virgin. In 2007, she began seeing Criminal Minds star Matthew Gray Gubler.In 2009, Dennings will appear in Shorts, a dark children’s film directed by Robert Rodriguez, and The Dream of the Romans, a story about a celebrity author whose manifestos become a sort of new Bible. Dennings hopes to make End Zone, a film based on the novel of the same name by Don DeLillo. Although actors Sam Rockwell and Josh Hartnett are involved, the project has yet to be greenlit because of it’s controversial subject matter, nuclear war.