Far from being a modern invention of the craft beer scene, pumpkin beers have a long history in the US. Samuel Stearns' The American herbal; or, Materia medica (published in 1801), name-checked pumpkin beer just after porter and ale. Stearns considered pumpkin beer especially healthful, noting:
Different kinds of beer, ale, &c. are often prepared according to the prescriptions of the physicians, all of which, as well as pumpkin and bran beer, partake of the virtues of the ingredients put into such liquors."
And before it was deemed a health tonic, pumpkin beer was a popular component in cups of flip—something akin to a cocktail that typically mixed rum, beer, and sugar. Pumpkin beer and brown sugar were more easily found in early America than their all-malt and refined counterparts, so they became part of the go-to recipe.
But the main reason pumpkin was adopted as a beer ingredient during the early colonial period was simple availability—pumpkins were a native plant (one completely unknown to most Europeans before the 16th century), while good malt was not so readily accessible—fermentable sugars had to be found where they could, and in the first pumpkin beers, the meat of the pumpkin took the place of malt entirely.
Indeed, the role of the pumpkin in brewing and as a means of general sustenance was a key subject of a satirical song that has become known as "America's first folk song"— first written in 1643, but rediscovered by folksong collectors of the 18th and 19th centuries:
Instead of pottage and puddings and custards and pies,
Our pumpkins and parsnips are common supplies;
We have pumpkin at morning and pumpkin at noon;
If it was not for pumpkins we should be undone
... Hey down, down, hey down derry down....
If barley be wanting to make into malt
We must be contented and think it no fault
For we can make liquor, to sweeten our lips,
Of pumpkins and parsnips and walnut-tree chips.
Others found the pumpkin's versatility a wonderful thing; in a history of Connecticut, first published in 1791 and aimed at a British audience that still had little knowledge of Things Pumpkin, noted that the pumpkin, (or pompion) could be used to make '...beer, bread, custards, sauce, molasses, vinegar, and, on thanksgiving day, pies, as a substitute for what the Blue Laws brand as antichristian minced pies.' And not just any beer, but 'good beer' at that.
Pumpkin beer continued to be a staple throughout the 18th century—one of the most oft-quoted recipes for pumpkin beer dates to 1771—but its popularity began to wane by the early 19th century as the pumpkin itself began to be viewed as something quaint and rustic, and as access to quality malts became commonplace. It re-appeared as a beer ingredient in the colonial revival of the 1840s (this time as a flavoring agent, as opposed to a full-blown pumpkin beer), but never regained its previous ubiquity.
Modern pumpkin beers tend to aim for more of a 'pumpkin pie in a glass' as opposed to 'pumpkin in a glass' aesthetic; spices such as nutmeg and cloves are very common ingredients—but where did the notion of reviving pumpkin beer originate? The honor goes to Buffalo Bill's Brewery, which has been making their America's Original Pumpkin Beer since the late 1980s, using one of George Washington's recipes as an inspiration. Although the experimental batches used pumpkin as an ingredient, the commercial version stuck with pumpkin pie spices instead (though there is now an Imperial Pumpkin Ale with actual pumpkin).
Other modern pumpkin beers do use pumpkins—Brooklyn Brewery's Post Road Pumpkin Ale evokes the 18th century in its name (using the name of the colonial road between Boston and New York) and includes pumpkin in the recipe, while Dogfish Head's Punkin Ale also adds pumpkin to the mix.
With more than 400 pumpkin beers to choose from today, modern drinkers may not be tasting anything like their beer's colonial ancestors, but it's still a nice (and now, a tasty) nod to brewing history.