He has collaborated with other graduates of the program, Christophe Beck and Marco Beltrami, writing additional music for Beck on the TV series Buffy the Vampire Slayer and other shows, and composing additional music for Beltrami on films including Scream 2, Scream 3, The Faculty, Scary Movie 2 and Resident Evil.
[2] Manthei began his game writing career working on titles such as Panzer General II, Vampire: The Masquerade - Redemption, Sacrifice, Wizardry 8, Twisted Metal: Black.
His other work on video game titles includes Kung Fu Panda, Marvel Universe Online, Upshift Strikeracer, Xiaolin Showdown, Ultimate Spider-Man, City of Villains, The Sims 2, StarCraft: Ghost, Shrek 2, Silent Hunter II, and Shark Tale.
For his work on Invader Zim, Manthei received an Annie Award nomination for Outstanding Achievement in Music for an animated series.
He wrote the music for the feature films Justice League: The New Frontier and Batman: Gotham Knight and scored the animated series Ultimate Spider-Man.