Source: Wikipedia

Disney had Ub Iwerks secretly begin animating a new cartoon while still under contract with Universal. The cartoon was co-directed by Walt Disney and Ub Iwerks. Iwerks was the main animator for the short, and reportedly spent six weeks working on it. In fact, Iwerks was the main animator for every Disney short released in 1928 and 1929. Hugh Harman and Rudolf Ising also assisted Disney during those years. They had already signed their contracts with Charles Mintz, but he was still in the process of forming his new studio and so for the time being they were still employed by Disney. This short would be the last they animated under this somewhat awkward situation.[19]

Mickey was first seen in a test screening of the cartoon short Plane Crazy, on May 15, 1928, but it failed to impress the audience and to add insult to injury, Walt could not find a distributor. Though understandably disappointed, Walt went on to produce a second Mickey short, The Gallopin' Gaucho, which was also not released for lack of a distributor.

Steamboat Willie was first released on November 18, 1928, in New York. 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 is considered by The Disney Company as Mickey's debut. Willie featured changes to Mickey's appearance (in particular, simplifying his eyes to large dots) that established his look for later cartoons and in numerous Walt Disney films.

The cartoon was not the first cartoon to feature a soundtrack connected to the action. Fleischer Studios, headed by brothers Dave and Max Fleischer, had already released a number of sound cartoons using the DeForest system in the mid-1920s. However, these cartoons did not keep the sound synchronized throughout the film. For Willie, Disney had the sound recorded with a click track that kept the musicians on the beat. This precise timing is apparent during the "Turkey in the Straw" sequence, when Mickey's actions exactly match the accompanying instruments. Animation historians have long debated who had served as the composer for the film's original music. This role has been variously attributed to Wilfred Jackson, Carl Stalling and Bert Lewis, but identification remains uncertain. Walt Disney himself was voice actor for both Mickey and Minnie, and would remain the source of Mickey's voice through 1946 for theatrical cartoons. Jimmy MacDonald took over the role in 1946, but Walt provided Mickey's voice again from 1955 to 1959 for The Mickey Mouse Club television series on ABC.

Audiences at the time of Steamboat Willie's release were reportedly impressed by the use of sound for comedic purposes. Sound films or "talkies" were still considered innovative. The first feature-length movie with dialogue sequences, The Jazz Singer starring Al Jolson, was released on October 6, 1927. Within a year of its success, most United States movie theaters had installed sound film equipment. Walt Disney apparently intended to take advantage of this new trend and, arguably, managed to succeed. Most other cartoon studios were still producing silent products and so were unable to effectively act as competition to Disney. As a result Mickey would soon become the most prominent animated character of the time. Walt Disney soon worked on adding sound to both Plane Crazy and The Gallopin' Gaucho (which had originally been silent releases) and their new release added to Mickey's success and popularity. A fourth Mickey short, The Barn Dance, was also put into production; however, Mickey does not actually speak until The Karnival Kid in 1929 when his first spoken words were "Hot dogs, Hot dogs!" After Steamboat Willie was released, Mickey became a close competitor to Felix the Cat, and his popularity would grow as he was continuously featured in sound cartoons. By 1929, Felix would lose popularity among theater audiences, and Pat Sullivan decided to produce all future Felix cartoons in sound as a result.[23] Unfortunately, audiences did not respond well to Felix's transition to sound and by 1930, Felix had faded from the screen.