The first appearance of Mickey (and Minnie) Mouse was the silent short “Plane Crazy”, made by Disney Brothers Studio in 1928 and shown to a test audience, but did not get a distributor.
The first-to-be-released Mickey Mouse film (which also included Minnie) was produced by Walt Disney Studios and distributed by Celebrity Productions Cinephone in 1928. It was the first sound-on-film animated picture. Following the success of “Steamboat Willie”, “Plane Crazy” was given a soundtrack and distributed by Celebrity Productions in 1929.
Google: Ub Iwerks.
Ub Iwerks and Mickey mouse.