Grant DeLong was born in Carlisle, Pennsylvania and began his ballet training with Marcia Dale Weary at the Central Pennsylvania Youth Ballet at the age of ten. He received numerous scholarships while attending Regional Dance America festivals. In 2001 and 2002, DeLong attended the School of American Ballet's summer programs on full scholarship.
DeLong joined American Ballet Theatre's Studio Company in September 2002 and the main Company as a member of the corps de ballet in April 2004. His repertory with the Company includes Lorenzo in Don Quixote, Demetrius in The Dream, Her Lover in Jardin aux Lilas, the Duke in Lady of the Camellias, Rakitin in A Month in the Country, the Spanish Dance in Alexei Ratmansky's The Nutcracker, Paris in Romeo and Juliet, a Celestial in Shadowplay, the Indian Prince and a Fairy Knight in The Sleeping Beauty, the Spanish Dance and the Neapolitan Dance in Swan Lake, Gonzalo in The Tempest and roles in Amazed in Burning Dreams, Company B, Dark Elegies, Drink to Me Only With Thine Eyes, Everything Doesn't Happen at Once, Gong and The Leaves are Fading. He created a role in One of Three.