The Principle of Mentalism does not come from ancient Egypt, although The Kybalion presents itself as though it does. The book appeared in Chicago in 1908 from the Yogi Publication Society, credited only to Three Initiates. Philip Deslippe's critical edition sets out the case that the author was William Walker Atkinson, a lawyer turned New Thought publisher who wrote under a long shelf of pen names and whose earlier books argue the same positions in the same voice. Other candidates have been proposed, Paul Foster Case and Michael Whitty among them, but no rival attribution carries comparable evidence.
That history matters most for this principle, because Mentalism is the one that reads least like Hellenistic Hermetism and most like the American New Thought movement the book grew out of. The surviving Greek Hermetica, the Corpus Hermeticum and the Asclepius, do set mind above the cosmos, and a reader of Copenhaver's translation will find nous in something like that position. What they do not contain is the practical turn Mentalism takes: the claim that because the universe is mental, a disciplined mind can act on it directly.
The atlas records the principle as it is stated and used, and separately records where it came from, because those are two different questions. Reading Mentalism as an ancient doctrine gets the history wrong. Reading it as a self-help slogan and nothing more gets its ambition wrong.
Sources The Kybalion: The Definitive EditionPhilip Deslippe (ed.) with Hermetica: The Greek Corpus Hermeticum and the Latin AsclepiusBrian P. Copenhaver