I'd say Irving Berlin, though many might not know the name, and certainly Francis Scott Key.
Regarding Stephen Foster, please see my old thread Who is Stephen Foster? (Now at 235 comments)
ML/NJ
Key wasn’t a composer.
Let me toss another name in the mix as a great American songwriter: Hank Williams. He wrote in a limited genre but in the late 40s and early 50s, country music (particularly Hank) was quite mainstream.
Some of us DO KNOW who “Jeanie With the Light Brown Hair” is. :-)