Ub Iwerks met Walt Disney in Kansas City in 1919 and there they formed a friendship and business relationship that would last for 47 years. Ub was still working at the Disney studios in 1966 when Walt passed away.
When all of Walt's animators jumped ship in 1927 to take his character of Oswald the Lucky Rabbit to Universal, Ub was the only one to remain at his old friends side. Single handedly animating the first 3 Mickey Mouse cartoons ("Plane Crazy", "Gallopin' Gaucho" and "Steamboat Willie"), Ub was clearly the artistic and technical force behind the birth of Walt's empire.
Ub also was instrumental (along with musical director Carl Stalling) in the Silly Symphony series, again single handedly animating the first one, "The Skeleton Dance".
Ub separated from Walt as well in the early 1930's, beginning his own animation shop which supplied cartoons to MGM and Columbia as well as other clients, but by the 1940's he was back home with Disney.
Ub was a technically brilliant man who also invented the multi-plane camera, which he built out of old Chevrolet parts, and brought that innovation back to Disney with him. He became the director of special optical effects or something similarly titled, and designed all the special effects used in Disney's live action films. Everything from the visual magic tricks used from "20,000 Leagues Under the Sea" to "The Shaggy Dog", from "The Absent-Minded Professor" to "Mary Poppins" sprang from the mind and hand of this man.
An early Silly Symphony by Ub:
One of my favorite Comi-Color cartoons he did. The Pincushion Man villain in "Balloon Land" is an all time favorite of mine:
Finally, for those who want to know more about this man (and you should) here's a documentary all about him called "The Man Behind the Mouse". Part one is posted here and you can just click through the YouTube links at the end to see the entire thing. This is also available on the "Walt Disney Treasures" DVD devoted to Oswald the Rabbit, something that's well worth owning!