Steamboat Willie was first released on November 18, 1928. It was co-directed by Walt Disney and Ub Iwerks. Iwerks again served as the head animator, assisted by Johnny Cannon, Les Clark, Wilfred Jackson and Dick Lundy. This short was intended as a parody of Buster Keaton's Steamboat Bill Jr., first released on May 12 of the same year. Although it was the third Mickey cartoon produced, it was the first to find a distributor, and thus has been cited as Mickey's debut.
-Taken from Wikipedia