Badelt was born in Frankfurt, West Germany. He started his musical career composing for movies and commercials in his homeland. In 1998, Oscar-winning film Composer Hans Zimmer invited Badelt to work at Media Ventures in Santa Monica, California, his studio co-owned by Jay Rifkin. Since then, Badelt has been working on a number of his own film and television projects such as The Time Machine and K-19: The Widowmaker. He also collaborated with other Media Ventures composers, such as Harry Gregson-Williams, John Powell, and Zimmer; and mentored several others like Ramin Djawadi and Steve Jablonsky.
Badelt co-wrote and co-produced the score to Hollywood box office hit Gladiator, directed by Ridley Scott, along with Zimmer and singer/composer Lisa Gerrard. Having contributed music to Gladiator, Mission: Impossible 2 and Michael Kamen's score for X-Men, Badelt was involved in the three most successful movies in 2000. Badelt also collaborated with Zimmer on other successful films, such as The Pledge, and 2001 blockbusters Hannibal and Pearl Harbor. One of his more famous and popular scores was for the 2003 film Pirates of the Caribbean: The Curse of the Black Pearl.
Among Badelt's most critically celebrated scores are the Chinese fantasy film The Promise and DreamWorks' remake of The Time Machine, the latter of which earned him the Discovery of the Year Award at the World Soundtrack Awards 2003. He also wrote the music for the closing ceremonies at the Beijing Olympics in 2008, and was commissioned to write an opera about China's First Emperor, to be premiered in 2015.
In 2004, Klaus founded his own film music company, Theme Park Studios, in Santa Monica. Since then, he has scored films such as Constantine, Poseidon, Rescue Dawn, and TMNT.
In 2013, Klaus founded a startup called Filmhub to create a B2B marketplace for content creators and digital streaming services.