At 21, she joined the Jimmy Dorsey band, replacing Helen O'Connell. One of her recordings with Dorsey was a favorite of American servicemen: "They're Either Too Young or Too Old" reached the No. 2 position in the Billboard charts in 1944. The same year, Kallen performed the vocals for Dorsey's number-one hit "Besame Mucho". Most of her singing assignments were in duets with Bob Eberly, and when Eberly left to go into the service toward the end of 1943, she joined Harry James's band.