Who Was The Mathematician Who Invented Pie?

Pi wasn't invented, it is a mathematical ratio which has been refined into ever more accurate detail by numerous societies. It is not definite which individual was the first to resolve an accurate number for pi - since many ancient civilisations were aware of this.E.G. The Egyptians thought of it as 22/7, and this is a good approximation.

The symbol π, which is the Greek letter (Pi), was not invented; it
would be more properly stated that Pi was discovered. Euclidean
geometry, attributed by Greek mathematician Euclid was the first
recorded system used to show Pi as a mathematical constant. Pi is
approximately equal to 3.14159, which is also referred to the constant circumference. William Jones used the name Pi for the Greek letter meaning the perimeter in 1706, and at a later time referred more often by Leonhard Euler.
Well if it had to be accredited to one person it would have to be Euclid.