Marianelli was born in Pisa, Italy. He graduated from the National Film and Television School in England. He composed the soundtracks for The Brothers Grimm (2005), Pride & Prejudice (2005), Atonement (2007), and Anna Karenina (2012); for the last three of these he received Oscar nominations for Best Achievement in Music Written for Motion Pictures, Original Score. In 2008, Marianelli won both an Oscar and a Golden Globe for the score of Atonement.
Marianelli has collaborated with the director Joe Wright four times, on Pride & Prejudice, Atonement, The Soloist, and Anna Karenina.