Here's the story:
* Early Years: In the early days of Mickey Mouse, Walt Disney *himself* voiced the character. He even did the voices for other characters like Minnie Mouse and Donald Duck.
* The Change: As Mickey became more popular, Walt Disney found it increasingly difficult to juggle his many responsibilities. So, he passed the voice acting duties to Clarence Nash.
* Clarence Nash: Nash is most famous for his long tenure as the voice of Donald Duck, but he also voiced Mickey Mouse for a period. He later voiced Mickey in some cartoons and radio shows.
* Later Voices: Over the years, several voice actors have lent their voices to Mickey Mouse, including Jim MacDonald, Wayne Allwine, and currently, Bret Iwan.
So while Walt Disney was instrumental in creating Mickey Mouse, he wasn't the one who provided his voice for very long.