Sorry, i didnt know which one it was. I feel so stupid.
Thanks though!

Here is pretty much everything known about his death:
But Mohatu never forgot the friends he had made there. He would occasionally visit Crocodile, where he would see hippos, zebras, and wildebeests at the river's shores. Mohatu ruled for a very long time, perhaps the longest of any king, and grew very old. But one winter's eve, he died. All his subjects wept for the loss of their king. Many felt lost without him and fighting arose among them again. But just as all hope was lost, a star appeared in the sky. Bigger and brighter than the rest. The animals grew peaceful and felt the same way they had when Crocodile had first let them drink from his great river. They knew it was the spirit of their king, who was still watching over them from the stars.

It is not said exactly how, but from what I can gather from this, he died of old age. Since it is nowhere mentioned otherwise. I am not saying it is not possible that he was just too old and became victim of some rogue lion or other predator. But that is up to you.

Also as said, he is grandfather of Mufasa. Mufasas father was Ahadi. But it is unknown if Mohatu is father of Ahadi or Uru(Mufasas mother).

1. Mohatu isn't Mufasa's father. He is either Ahadi or Uru's father.

2. Old age presumably.