The fruit either rots or birds/other animals eat the juicy parts. Then there's the seed.
Mango seeds were likely dispersed by extinct megafauna that once roamed the earth's surface. They are an example of what is called "evolutionary anachronism" which basically means that there is no extant (that's the opposite of extinct!) animal that is capable of ingesting and dispersing the seed. Interesting huh!? Avocados are another example of a fruit occupying a similar ecological niche.
Source:
PhD Evolutionary Biology
Source:
PhD Evolutionary Biology
By animals
A mango seed can be dispersed by I don't know
By air