His father was Harold Edward Meyer, a banker, and his mother, Berniece Graf, was formerly Miss Nebraska and a Ziegfeld girl.

During his Army days, Janssen became a friend of fellow enlistees Martin Milner and Clint Eastwood while posted at Fort Ord, California.

[11] His films include: To Hell and Back, the biography of Audie Murphy, who was the most decorated American soldier of World War II; Hell to Eternity, a 1960 American World War II biopic starring Jeffrey Hunter as a Hispanic boy who fought in the Battle of Saipan and who was raised by Japanese-American foster parents; John Wayne's Vietnam war film The Green Berets; opposite Gregory Peck, in the space story Marooned, in which Janssen played an astronaut sent to rescue three stranded men in space; and The Shoes of the Fisherman, as a television journalist in Rome reporting on the election of a new Pope (Anthony Quinn).

Janssen's impressively husky voice was used to good effect as the narrator for the TV mini-series Centennial (1978–79); he also appeared in the final episode.

Though Janssen's scenes were cut from the final release, he also appeared as a journalist in the film Inchon, which he accepted to work with Laurence Olivier, who played General Douglas MacArthur.

At the time of his death, Janssen had just begun filming a television movie playing the part of Father Damien, the priest who dedicated himself to the leper colony on the island of Molokai, Hawaii.

Milton Berle, Johnny Carson, Tommy Gallagher, Richard Harris, Stan Herman, Rod Stewart, and Gregory Peck were among Janssen's pallbearers.