There, Hendricks began his singing career at the age of seven. He has said: "By the time I was 10, I was a local celebrity in Toledo. I had offers to go with Fats Waller when I was 12, and offers to go with Ted Lewis and be his shadow when I was 13. He had that song 'Me and My Shadow'. And he had this little Negro boy who was his shadow, that did everything he did. That was his act."
As a teenager, Jon's first interest was in the drums, but before long he was singing on the radio regularly with another Toledo native, pianist Art Tatum. Jon met his first wife Colleen Moore in Toledo, Ohio. They were married and had 4 children.
After serving in the Army during World War II, Hendricks went home to attend University of Toledo on the G.I. Bill as a pre-law major. Just when he was about to enter the graduate law program, the G.I. benefits ran out. Charlie Parker had, at a stop in Toledo two years prior, encouraged him to come to New York and look him up. Hendricks moved there and began his singing career.
In 1957, he teamed with Dave Lambert and Annie Ross to form the legendary vocal trio Lambert, Hendricks & Ross (LH&R). With Jon as lyricist, the trio perfected the art of vocalese and took it around the world, earning them the designation of the "Number One Vocal Group in the World" for five years in a row from Melody Maker magazine. Their multi-tracked album Sing a Song of Basie was one of the earliest examples of overdubbing.