The most technically correct answer is Walt Whitman.

Longfellow got dismissed by several people here for being too sentimental - but I would argue that sentimentality and saccharine character is distinctly part of America's mythos.

Washington Irving and Parson Weems would both also be good candidates. Early writers who had a penchant for mixing real history with tall tales.

The name I would throw out is John Ford. Ford basically invented the modern action movie, the mythology of the West, redefined masculinity, established the tropes of a hero, etc.