Did Marie Antoinette say let them eat cake?
Almost certainly not. The phrase predates her prominence, was attributed to others before her, and appears in no contemporary source connecting it to her.
The original is French: Qu'ils mangent de la brioche — "let them eat brioche", an enriched bread rather than cake. The line appears in Jean-Jacques Rousseau's Confessions, written around 1765, where he attributes it to "a great princess" without naming her. Marie Antoinette was about ten years old at the time and still living in Austria; she did not arrive in France until 1770.
Versions of the same anecdote circulated earlier still, attached to various royal women, which suggests a travelling story rather than a recorded remark.
It appears to have been attached to Marie Antoinette decades later, during and after the Revolution, when she was the target of an enormous volume of hostile pamphleteering. She was foreign, associated with an unpopular alliance, and blamed for royal extravagance during genuine food shortages. A great deal of what was written about her in that period was propaganda, some of it extremely crude.
The historical record suggests she was, if anything, moderately charitable by the standards of the court, though also genuinely extravagant in ways that made her an easy target.
Whether she said it has arguably become less important than what it represents: the phrase endures as shorthand for rulers who have no comprehension of how ordinary people live, and it is used that way regardless of its origin.
She was executed by guillotine in October 1793.
A related detail often attached to her is also doubtful: the claim that her hair turned white overnight before her execution. Contemporary accounts describe her hair greying during her long imprisonment, which is considerably less dramatic and considerably more plausible.