I knew a tipping point of sorts had been reached last week, when I was at the gym of the small liberal arts college in my town. (I have gym privileges because my wife works at the college.) Two undergrads, one of them sitting at a weight machine, were chatting. There was a pause, then the other said, “I’ll leave you to it,” and walked away.
That phrase, “I’ll leave you to it,” had been on my NOOB-dar for a while. I had a sense of it as a British thing, yet characters recently used it on the American sitcoms Midcentury Modern and Abbott Elementary. A 2023 New York Times cooking column (unsigned but undoubtedly written by NOOBs favorite Sam Sifton) closed with the paragraph:
“Or maybe you’d like something decidedly soupy? Yotam Ottolenghi’s herby sweet potato soup with peanuts is also substitute friendly, like the sancocho, with added carby comfort from bulgur or short pasta. Yotam calls it a soup for when you just want to be left alone,’ so we’ll leave you to it.”
As for British origin, some research backed my hunch up. The OED actually has a definition for the phrase, which, I learned, doesn’t necessarily have to begin with the word “I’ll”:
“to leave (a person) to it: to leave (a person) alone to proceed with a task in hand; to allow to get on with something without interference.”
The dictionary’s citations–which go back a surprisingly long time–are all British:
(BTW, the “S. Mather” in the 1671 quote isn’t one of the American preaching Mathers, but Samuel, an English minister.)
Google Ngram viewer adds a little more nuance.
That is to say, the expression has always been more common in Britain the the U.S., but it shot up in popularity in both countries in the decade of the 2000s, to the point of becoming a catch phrase.
Thanks for reading. Now, I’ll leave you to it.
notoneoffbritishisms.com 
i remember hearing the phrase used on an American sitcom long ago, probably in the 90s at the latest. Wish I could remember which, but I’m going to guess it was Frazier. Or Frasier.
Very interesting. I think of this as a British expression unless it’s used very literally. I can imagine an American boss sticking her head in the door to talk to a subordinate. The subordinate says she’s in the middle of something she is concentrating on, so the boss says, “Got it. I’ll leave you to it.” But any more casual, less literal, usage strikes me as inherently British.
I’d say – as a British English speaker – that it’s a way of closing a conversation without appearing awkward, or seeming rude for just walking off without a word. I work in a public library, and might well use the phrase if I’d been helping a customer choose books or to use the computers. It kind of carries the unspoken promise that they can ask for further help if they need it.Eleanor Sutton[I did try to post this as a comment, but it didn’t like my password, and then I think it lost what I’d just typed.]
I agree, as another British English speaker. And it sounds like the American gym-goers in Ben’s anecdote have picked up the phrase with exactly the same connotations it has for us!
I have always used this as a kind of escape ladder. If I want to break away from an encounter and the other person doesn’t read my foot shuffling, staring at the door and hint dropping, I might say, “O.K., I’ll leave you to it.” and go. To suggest that I am doing them a favour by freeing them to continue with their task feels less abrupt; more as if I am leaving to serve them rather than myself. Sometimes it’s hard to be British.