Kiley had had a respectable career on Broadway -- he'd starred in the minor success, Redhead, opposite Gwen Verdon, for which he won the Tony Award as Best Actor in 1959. But it was Man of La Mancha seven years later (for which he won another Tony).
Most of his work after that was in television, most notably starring in the series A Year in the Life (for which he won an Emmy Award), and the mini-series The Thorn Birds (which won him another Emmy). In all he won three Emmy Awards. But in addition to TV movies and a lot of guest-starring, there also was a huge amount of voiceover narration work on documentaries. Because of that voice.
And here is that voice that got him there.