Mickey was born to a family of mice in 1903. He was tested on by Albert Einstein for several years as a lab mouse, but was thrown out after he supposedly died. He, in fact, did not die. He mutated into a weird mouse creature.
Mickey wondered the streets (until the Police showed up) looking for work. He eventually retreated to the poorest neighborhood he could find: an ally in Hollywood. A bunch of TV dudes found him, and made him in TV shows. This made Mickey trusted by the world.