If it's squirrels, I've got a bit of a handle on them now. You just have to patrol (and shake!) the tree first thing in the morning, and every couple of hours through the day and pick up any fruit that they drop to the ground.
Raccoons are a much harder problem - since they come at night. I had some success with spraying the fruit with hot pepper but it seems like they got used to the taste. At least they eat the whole fruit when they take one.
I should loan you my motion detector sprinkler to see if it does any good for you. I can't use it in the back yard because it gets the cat every time and she comes in very p'd off. I do use it in the front yard for my lychee tree, though. Usually the lychees are done before the mango tree really gets going.
