He remained with the band until it dissolved in 1942, then began working as session musician, with much of his time spent at Decca Records. He recorded with Billie Holiday, Duke Ellington, Benny Goodman, and Ella Fitzgerald; his arrangements can be heard on Fitzgerald's album Lullabies of Birdland.
In 1935, Haggart became a member of the Bob Crosby Band. He arranged and composed "Big Noise from Winnetka", "My Inspiration", "What's New?