There's actually no single person who invented French Toast.
However French Toast is thought to have been invented and originated in Ancient Rome.
The earliest known recipe for what resembles the French Toast that we know today is found in Apicius, which is a 1st century, CE cookbook from ancient Rome.
The Roman Recipe for French Toast which is called Aliter Dulcia which is another sweet dish, involves soaking pieces of white bread in milk and beaten egg and frying them and covering them with some honey.
The concept of French Toast also then spread throughout Europe.
And in 14th century Germany, the French Toast dish was known as Arme Ritter, or Poor Knights.
And in 15th century France, a similar dish was also called pain perdu, which means lost bread, is a term that reflected the practice of using stale bread to prevent waste.
And despite the name French Toast, the French Toast dish is actually not French in origin, but was popularized in the United States, by French immigrants.
And Americans adopted the name French Toast for the dish as it was brought to American by French immigrants and become very popular there.
In France French Toast is called pain perdu and in Germany and some Nordic countries, French Toast is called Arme Ritter.