Composers have needed arrangers for Broadway as far back, as I can tell, to 1902 and the Wizard of Oz, although I sure that for some time before that it wasn't an uncommon occurrence for a music publisher to have a house arranger harmonize a tune sung, hummed or whistled by a "composer," who got full credit for the piece whilke the actual writer was paid a fee.
Mabel Barrison claimed that the hit "Sammy," her big song in Wizard of Oz was her composition; she sang the lyrics she'd written to a pianist in a Chicago music store who notated and harmonized it for someone to orchestrate for the June 1902 opening. The song was published as a song by James O'Dea (lyricist Anne Caldwell's husband) and Ed. Hutchinson.
The most famous non-composer of the 1950s was Bob Merrill who played his tunes on a toy xylophone and the work was filled out by a professional arranger.