D'Elia was born in Clifton, New Jersey, to a mother who works in printing, and Anthony D'Elia, a lawyer. Her father is of Italian descent and her mother is Jewish. She started acting at the age of 5 and signed up for dancing classes at Broadway Bound. When she was seventeen, she was cast in her first major gig on the soap opera All My Children as the recurring character Bailey. In 2010, she booked the role of Tea on the MTV adaptation of the hit British teen drama Skins, where she plays an openly lesbian teenager. She is featured in Hoodie Allen's music video for "The Chase is On" and plays Allen's love interest in the video. As of fall 2011, D'Elia had begun classes at The William Esper Studio as well as landing the role of Mina in Vicky Jewson's Born of War. This is her first film role. Starting in the fall of 2012, D'Elia played Sage Spence in the sixth and final season of Gossip Girl.