Harrell is a prolific arranger and composer. He has arranged for Vince Guaraldi's work on "Peanuts", Carlos Santana, the Metropole Orchestra, the Danish Radio Big Band, the Vanguard Jazz Orchestra, and Elisabeth Kontomanou with the , among others. His compositions have been recorded by other jazz artists including Ron Carter, Kenny Barron, Art Farmer, Chris Potter, Tom Scott, Steve Kuhn, Kenny Werner and Hank Jones. Harrell's composition and big band arrangement entitled "Humility" was recorded on the Grammy-winning album by Arturo O'Farrill's Afro-Latin Jazz Orchestra, "Song for Chico". As a composer and arranger, Harrell works in different genres, including classical music.