He is best known for his scores in films such as 500 Days of Summer (2009), The Age of Adaline (2015), Gifted (2017), The Only Living Boy in New York (2017), Tully (2018), Love, Simon (2018), The Front Runner (2018), The Way Back (2020), Ghostbusters: Afterlife (2021), The Adam Project (2022), The Whale (2022), Deadpool & Wolverine (2024), and It Ends with Us (2024), and the television series Life in Pieces (2015–2019).
In 2004 he teamed up with composer Mychael Danna and has provided additional music and arrangements on Surf's Up, Fracture, Moneyball, and the Oscar-winning Life of Pi.
Simonsen and Danna co-wrote the score of (500) Days of Summer, and Joss Whedon's television series Dollhouse.
Simonsen's work includes the Sundance Film Festival features The Way, Way Back and The Spectacular Now;[2] the Lionsgate feature film The Age of Adaline; and most recently Love, Simon, The Way Back, and Stargirl.
In 2009 he opened his own studio, and The Hollywood Reporter named him one of the "15 Composers Primed to Take Their Place on the A List.