Linguists believe that around the turn of the 19th century, the then-English accent began conspicuously diverging from its American counterpart. The main reason behind this split was rhotacism. Traditionally, English, whether spoken in the Old World or the New World, was rhotaic. This means that its speakers pronounced the /r/ when it occurred immediately after a vowel and was not followed by another vowel.
In his 1887 short story, The Canterville Ghost, Oscar Wilde famously stated, “[w]e have really everything in common with America nowadays, except, of course, the language.” But the truth, as it turns out, is stranger than fiction.
Most people incorrectly assume that the English colonists gained a different accent in the subsequent years of founding America. Before the American Revolutionary War, British English sounded more like the General American accent of today. Instead, it was the British who gained a ‘British accent.’
Of course, there is no one, singular British accent more than there is one lone American accent. The Anglo-Celtic Isles have a mindboggling 56 accent-types, including Scottish, Welsh and Irish accents. London alone has three distinctive accents. Regional variations and socio-economic class are obviously determining factors here (Remember Eliza Doolittle’s very uncouth Cockney accent?). Hence, the phrase ‘British accent’ is a gross misnomer in itself.
Linguists believe that around the turn of the 19th century, the then-English accent began conspicuously diverging from its American counterpart. The main reason behind this split was rhotacism. Traditionally, English, whether spoken in the Old World or the New World, was rhotaic. This means that its speakers pronounced the /r/ when it occurred immediately after a vowel and was not followed by another vowel. A rhotaic English speaker pronounces the ‘r’ in the word ‘hard,’ while a non-rhotaic speaker does not, leaving it sounding more like ‘hahd.’
However, sometime after the American Revolution a major chunk of the upper classes in southern England stopped speaking rhotaically. This emergent accent rapidly became a signifier of sophistication and fine breeding. It was increasingly patronized by teachers of elocution. Emulating the aristocracy, the lower classes (with their aspirations for upward social mobility) began cultivating this non-rhotaic articulation too. Eventually, this posh accent became standardized as ‘Received Pronunciation’ or BBC English. Because of its ‘neutrality’ and ease of understanding, it spread across the rest of Britain. Hence, the modern British accent was born.
Since, the British cultural and linguistic influence on the American colonies was waning after its political defeat in the American Revolutionary War, the Americans largely retained their rhotic speech. However, regional exceptions to both these general American and British accents still remain. People in southeastern England, north England, Scotland, and Ireland still speak in their traditional rhotic accents. Similarly, there also exist American dialects with non-rhotic features like Southern English, Boston English, and the Virginia Tidewater accent. The British elite exercised the maximum influence on port cities that had close trading ties with England. This explains the non-rhotic accents of Boston, Savannah, Charleston and Richmond.
Language is not a static entity. Linguistic evolution as a rule is a gradual but constantly ongoing process. So it follows that both accents must have evolved further since this rhotaic shift. Both resulted from different continua of change. Just that the English accent did so more pronouncedly and less subtly in the last two centuries. The American accent itself was informed by the huge immigrant influx. The Dutch, the French, the German…an assortment of nationalities were ultimately influential in forming the American English as we know it today.
As to what exactly the original accents of the Pilgrim Fathers were like when they first came to the Americas in the early seventeenth century? Nothing definitive is known. What we do know is that the present General American accent is much closer to the way the English spoke at the time.