An article on Slate.com explains that sweet dessert pies are a fairly recent invention:
“We eat sweet pie at Thanksgiving on the premise that it captures the cuisine of colonial America. It does nothing of the kind. Sweet pie didn’t gain wide popularity until the 19th century… (in A Tramp Abroad, Mark Twain included sweet pies in a list of things he missed about his homeland…), and the full American pie menu, in all of its moods and seasons, did not come into being till the 20th. American as apple pie, the phrase and concept, entered our lexicon in the late and cosmopolitan throes of the Jazz Age. The most American thing about pie, in fact, may be its retroactive claim of folksy authenticity and early dominance.” The apple’s status as an immigrant seems appropriate since it mirrors most of America’s families.
Today I Found Out notes that “the first recorded recipe for apple pie was written in 1381 in England, and called for figs, raisins, pears, and saffron in addition to apples.” But no sugar – a rare and expensive ingredient – and the pastry was used as a baking container, not to be eaten. “A recipe for apple pie very similar to today’s recipes appeared in a Dutch cookbook in 1514.” But the apple pie hadn’t made it to America yet.
Skipping ahead to World War II, a common soldiers’ slogan was “‘for mom and apple pie’ which later gave rise to ‘as American as motherhood and apple pie’. Along with Phrase Finder, Today agrees that “apparently in the 1960s, we began to be ‘as American as apple pie.'”
I found one outlier. Kelly Kazek posted that the phrase “was in use by the 1860s, leading to its place in history as an American favorite,” but – alas – she lists no citation. Could “1860s” be a typo? Her phone number is on the web page, though, if you’d like to pester her.