[4] As a youth, he was recognized as an extraordinary talent and formed a traveling duo with Danny Polo, a musical prodigy on the clarinet and trumpet from nearby Clinton, Indiana.

He and clarinetist Artie Shaw started their careers at the Golden Pheasant in Cleveland, Ohio, with the Austin Wylie Orchestra.

Thornhill went to the West Coast in the late 1930s with the Bob Hope Radio Show and arranged for Judy Garland in Babes in Arms.

Thornhill was playing at the Paramount Theater in New York for $10,000 a week[citation needed] when he enlisted in the U.S. Navy as an apprentice seaman on October 26, 1942.

Polo, Gerry Mulligan, and Barry Galbraith returned with new members, Red Rodney, Lee Konitz, Joe Shulman, and Bill Barber.

[citation needed] Thornhill and his orchestra played for the Judy, Jill and Johnny variety program on radio on the Mutual Broadcasting System in 1946–1947.

Thornhill's compositions included the standard "Snowfall", "I Wish I Had You", recorded by Billie Holiday and Fats Waller, "Let's Go", "Shore Road", "Portrait of a Guinea Farm", "Lodge Podge", "Rustle of Spring", "It's Time for Us to Part", "It Was a Lover and His Lass", "The Little Red Man", "Memory of an Island", and a big band arrangement of the folk song "Where Has My Little Dog Gone?"