The Law of Attraction is the best-known thing New Thought has produced, and its lineage is documented well enough to set out plainly.
The premise is that thought is causal: that sustained attention to a desired outcome tends to bring it about, and that attention to a feared one does the same for the fear. The mechanism offered has shifted with the science of the day. Nineteenth-century writers spoke of mind acting directly on mind and on matter; twentieth-century ones borrowed the vocabulary of vibration and resonance; recent presentations reach for quantum mechanics, which does not support them and is not, on any reading of it, about what a person wants.
The line runs like this. Phineas Parkhurst Quimby, working as a mental healer in Maine from the 1840s to the 1860s, held that illness follows from mistaken belief and is cured by correcting the belief. Prentice Mulford, in essays collected as Thoughts Are Things from 1889, gave the doctrine its popular shape and a good deal of its later vocabulary. William Walker Atkinson published Thought Vibration, or the Law of Attraction in the Thought World in 1906, which is where the phrase itself becomes a title. Wallace D. Wattles wrote The Science of Getting Rich in 1910, applying the teaching to money specifically and without embarrassment.
Its twenty-first-century revival is a single identifiable event rather than a drift. Rhonda Byrne released the film The Secret in 2006 and the book of the same name that year; Byrne has said that Wattles's book was her starting point. It sold in the tens of millions and returned the doctrine to mass circulation with most of the older religious framing stripped away. What it added was emphasis. The earlier writers coupled mental discipline to work, service and character, and were often quite specific about the work. The Secret presents the mental act as very nearly sufficient by itself.
The through-line from Quimby to Byrne is continuous, American, and roughly a hundred and eighty years long. It is not ancient. It is not Hermetic in origin, whatever the later borrowing of as above, so below suggests. And it is not a suppressed teaching recovered against opposition, which is the claim the title makes and the claim that sells it.