Reddit mentions of Milestones in the History of English in America (Volume 76) (Publication of the American Dialect Society, No. 86)

Circumstantially, an American accent of the English language began to arise just as soon as there were American born children in Virginia in the 1610s and in New England in the 1620s.

Even in the early days, the English colonists came from towns all over England, so their children spoke at first with some combination of whatever accents were around in their youth. For the first couple decades at least, immigrant children still outnumbered American-born children, and, of course, the adults were all immigrants, too, until the first American-born children grew up. So it wasn't until the mid-1600s before a sizable population of American-born English speakers had reached adulthood in Virginia, Maryland, and the New England colonies. (The other colonies either weren't founded, or weren't under English control, until 1663 or later.)

Through analysis of compiled immigration data of the era, historian Paul K. Longmore estimates that around the year 1670, enough of the northern Virginia/Maryland area was populated, and enough of the Massachusetts Bay and surrounding area was populated, and enough of that population was American-born, that a true American way of speaking the English language would have emerged. An accent would have been recognizable to British English visitors upon arrival in America.

One piece of evidence that backs this analysis up is that around this time, British writers began writing about American misuse of words. The earliest such mention comes from An Account of Two Voyages To New England by John Josselyn, published in 1674. In the book, Josselyn recounted a visit to Boston in 1663 where he noticed Americans used the word "ordinary" the wrong way. In 1663 England, an "ordinary" was a place to get drinks and food, while in 1663 Boston, an "ordinary" was a place that only served drinks.

In the next few decades, the evidence for the emergence of an English accent became even clearer. The earliest direct mention of an American accent was by Hugh Jones, an England-born professor at the College of William and Mary, who arrived at the school in ~1717 and taught there until 1721. After his return to Great Britain, he wrote a book called The Present State of Virginia, published in 1724, which described the American way of speaking:

> "[In Virginia, t]he Planters, and even the Native Negroes generally talk good English without Idiom or Tone [i.e. without slang or an accent], and can discourse handsomely upon most common Subjects..."

Yet, Jones preceded this sentence with a self-diagnosis of Virginia's accent. Jones stated that Virginians "esteem" London "their home", and looked down upon the manners of "Bristol, and the other Outports" due to "seeing and hearing the common Dealers, Sailors, and Servants that come from those Towns, and the Country Places in England and Scotland, whose Language and Manners are strange to them."

Hugh Jones' description of Virginia accents coincides with the years that Benjamin Franklin was growing up in Massachusetts. He was born in Boston in 1706 to an English father, and lived there until the age of 17, in 1723, when he moved to Philadelphia.

Fifty years later, in 1773, Franklin was a newspaper publisher caught up in a media scandal called the Hutchinson Letters Affair. The then-Governor of the Massachusetts colony, Thomas Hutchinson, had leaked some letters to Ben Franklin who then gave the letters to the Boston Gazette. There was a trial back in London over the leak, and during closing arguments, the prosecutor lambasted Franklin for the leak and called attention to his accent:

> "After the mischiefs of this concealment had been left for five months to have their full operation, at length comes out a letter, which it is impossible to read without horror; expressive of the coolest and most deliberate malevolence.—My Lords, what poetic fiction only had penned for the breast of a cruel African, Dr. Franklin has realized, and transcribed from his own. His too is the language of a Zanga:
>
> " Know then 'twas — I.

>" I forg'd the letter — I dispos'd the picture—

>" I hated, I despis'd, and I destroy."

Though the prosecutor is no doubt embellishing for effect, he seems to be mocking Franklin's "common" speech attributes, and calling him a foreigner. It's safe to assume that whatever American accent that Benjamin Franklin had, he picked it up in 1710s Boston, just about the same time that the American accent was first being written about down in Virginia.

One of the leading authorities on the subject of the emergence of the American accent was etymologist Allen Walker Read, who published a number of articles around the topic beginning in the 1930s. His articles collected dozens of instances where a distinct American accent was mentioned or was hinted at during the colonial perod. For instance, he collected a number of mentions by English visitors in the 1700s commenting on the prolific use of profanities and obscenities in American conversation. The earliest mention comes from a 1699 travel book called A Trip to New-England by Ned Ward:

> "Notwithstanding their [New Englanders'] sanctity, they are very prophane in their common Dialect."

On its own, the comment isn't much evidence of anything, but taken as the first in a series of such notices throughout the 1700s, an emerging American trend can be identified. Similarly, through collecting dozens of descriptions of runaway English-born indentured servants, Read found that Americans were talking about "English", "Yorkshire", and "West Country" accents as early as 1721:

>"Run away...a Servant Man, named William Newberry...He is a West-country-Man, and talks like one." American Weekly Mercury (Philadelphia), Jun 1, 1721
>
>"Run away....two Servant Men, the one...a Yorkshireman who talks very broad." American Weekly Mercury, Jul 18, 1734
>
>"Ran away...a Servant Man, named John Smith...an Englishman, and speaks very plain." Virginia Gazette, Oct 24, 1745

By mid-century, it started to become commonplace for British visitors in their writings to comment on the distinctive American accent. And they usually had one of two things to say about it: that Americans often spoke English in a more "proper" or else old-fashioned accent than did English people themselves, or that the Americans had a striking lack of regional variation in accents from one colony to the next, at least in comparison to England. Among typical comments on American accents of the era is this passage from a letter dated June 8, 1770, and written by an Englishman named William Eddis:

>"In England, almost every county is distinguished by a peculiar dialect...but in Maryland and throughout the adjacent provinces [colonies], it is worthy of observation, that a striking similarity of speech universally prevails; and it is strictly true, that the pronunciation of the generality of the people has an accuracy and elegance, that cannot fail of gratifying the most judicious ear...

To this day, it remains true that there is much more regional variety of accents in the U.K. than in the U.S., so it's not entirely clear if this is what foreign visitors to the Thirteen Colonies were noticing. Those who stayed long enough in America, and those American-born, tended to notice regional accents more readily. By the time of the American Revolution, distinct regional accents in America began to be commented upon with some regularity:

>
>"Though the inhabitants of this Country are composed of different Nations and different languages, yet it is very remarkable that they in general speak better English than the English do. No Country or Colonial dialect is to be distinguished here, except it be New Englanders, who have a sort of whining cadence that I cannot describe." - Journal of Nicholas Creswell, Jul 19, 1777

And:

>..."[The English colonies] in America trace their original to a few active cities, London, Chester, Bristol, and the like; whose phrases and accents are yet discoverable in the speech of the colonists. In Virginia, one of the oldest of the British settlements, we still hear such terms as holp for help, mought for might; and several others now obsolete here, but which were in full currency at the time when that colony was first planted." - Reminiscenses of an American Loyalist, 1738-1789, by Jonathan Boucher, first published 1832

In the post-war period, differences between American and British dialects were written about ever more often. In 1806, Noah Webster began work on an American dictionary of the English language.

TL;DR: Circumstantially, the first American accents were heard just as soon as there were American-born children old enough to talk in 1610s Jamestown and 1620s Plymouth. There is indirect evidence of accents between the 1660s and 1710s, while the first direct mention of an American accent comes from a 1717-1721 visit to Williamsburg, Virginia, by a British-born professor. By the 1770s, the American accent had been regularly commented upon, often remarking on different dialects heard in New England, the Middle Colonies, and Virginia/the South.

Further Reading:

Start with Milestones in the History of English in America by the aforementioned etymologist Allen Walker Read

"Good English without Idiom or Tone": The Colonial Origins of American Speech by Paul K. Longmore, as referenced multiple times above

Imperatives, Behaviors, and Identities: Essays in Early American Cultural History by Jack P. Greene, which contains more scholarly essays