With the release of a sound cartoon titled "Steamboat Willie" at the Colony Theater in New York way back in 1928, a whistling Mickey Mouse was introduced to theater going audiences.

Mickey wasn't Walt's first cartoon creation.
Oswald the Lucky Rabbit starred in many different animated cartoons before Walt lost the rights to the rabbit in 1927.

And the rest, as they say, is animated history. I'm sure Mickey and the Gang will still be entertaining audiences in one form or another for many more years.