At age 16, he left home to join traveling big bands, playing with Tommy Dorsey, Benny Goodman and Ray Noble.

Hagen was an orchestrator and arranger for motion picture studio 20th Century Fox in the 1940s and early 1950s, and worked on films like Call Me Madam, Gentlemen Prefer Blondes and Carousel.

Hagen met television show producer Sheldon Leonard when he scored the Danny Thomas series Make Room for Daddy.

Sheldon Leonard, the producer and creator of I Spy, bucked the trend of using canned music for television shows and instead decided to create original soundtracks for every episode.

Since every episode of I Spy was set in a different location, Hagen made liberal use of world music in his soundtracks which were mostly written and performed within the West coast jazz genre.

[3] Two of his students were fellow Emmy-winning composers and orchestrators Bruce Babcock and Harvey Cohen[6][7] Hagen was married for 59 years to Elouise "Lou" Sidwell, a former big-band singer, until her death in 2002.