Which David Fincher film was the first to be scored by Trent Reznor?

The David Fincher film first scored by Trent Reznor was The Social Network.

Released in 2010, The Social Network marked Reznor's first feature-film scoring collaboration with Fincher. Reznor composed the music with Atticus Ross, creating a tense electronic score for Aaron Sorkin's dramatized account of Facebook's beginnings and the lawsuits surrounding Mark Zuckerberg.

The collaboration began after Fincher approached Reznor, who initially declined because he had recently completed an exhausting touring and recording schedule with Nine Inch Nails. He reconsidered after reading the screenplay and became deeply involved in shaping the film's sound. The score's cold synthesizers, distorted textures, and understated melodies reflect the film's themes of ambition, alienation, and technological disruption.

A common mix-up is assuming The Girl with the Dragon Tattoo was their first Fincher project because it followed soon afterward in 2011. In fact, Reznor and Ross reunited for that film, then scored Gone Girl, Mank, and The Killer. The Social Network score won both the Golden Globe and the Academy Award for Best Original Score.