Mortimer Mouse is a fictional character created by Floyd Gottfredson for The Walt Disney Company. He is Minnie Mouse's ranch-owning cattleman uncle. He first appeared in the comic strip Mickey Mouse in Death Valley (1930). After that, he appeared in many Mickey Mouse comic strip adventures in the early 1930s.
The character named Mortimer Mouse, Mickey's competition for Minnie's affections, was introduced in the 1936 cartoon short Mickey's Rival. In the comics, this Mortimer was briefly renamed Montmorency Rodent, in an attempt to differentiate him from the pre-existing uncle, but the new name didn't stick. Mickey's rival was once again Mortimer in later comics—and in the animated series Darkwing Duck, Mickey MouseWorks, and House of Mouse, where he is known for his catchphrase, "Ha-cha-cha!"
In House of Mouse and Mickey MouseWorks, Mortimer as Mickey's rival is voiced by Maurice LaMarche. As Minnie's boss in Mickey's Once Upon a Christmas, he was also voiced by LaMarche, but with a different inflection. Minnie's Uncle Mortimer has never appeared in animation.
Outside of Disneydom, "The Adventures of Mortimer Mouse" is also a children's book by Charlene McCoy.
Mortimer Mouse was also the name of a character in Berkeley Breathed's comic strip Outland. He is a surly, stubbly, chain-smoking mouse who wears two-button shorts and has a bite taken out of one ear as a result of a scuffle with ex-Disney CEO Michael Eisner.