Mortdecai (commonly known as Mort) is a main protagonist in All Hail King Julien. He is an incredibly cute and somewhat innocent and annoying mouse lemur, although his age is not confirmed. (It was revealed in All Hail King Julien that he is at least more than 50 years old. He is old enough to be in history books of Madagascar and the lemur kingdom.) He is a side character in Madagascar, Madagascar Escape 2 Africa, and Madagascar 3: Europe's Most Wanted, a side character in Penguins of Madagascar (film), a major character in The Penguins of Madagascar (series), and the tritagonist of All Hail King Julien. He loves King Julien's feet, an obsession which started when he saw King Julien using them to kick some foosas in the first episode of All Hail King Julien. In The Penguins of Madagascar, he is somewhat cute toward the other zoo animals.

Mort's love for feet is so extreme that, while sleeping, he can rip down a tree to get to them, as seen in "Two Feet High and Rising." King Julien also hates it when Mort touches his feet, so he kicks him away. Mort is protected by a "force field" of ignorance and can't feel pain, but he still hates drowning, as seen in "Assault and Batteries." He laughs hysterically a lot. (In Madagascar: Escape 2 Africa, while being chased by a shark and laughing crazily, while at the same time, he says "Why am I laughing?!?") He likes things even if he doesn't know what they are. He says "I like [fill in]!". (i.e. In "Tangled in the Web," he says "I like computers!" despite not having a clue as to what a computer is, and claims "I don't have to know to like.")

In All Hail King Julien: Exiled, it is revealed that Mort is of a species that can absorb other Morts in the multiverse (or, MORTiverse). However, Smart Mort, and later “Groovy 60's cult leader” Mort, also known as Hippie Mort, are two alternate versions of Mort, created by him due to the effects of both coffee, and tea (also known as Brown Julien, and Golden Julien) in both One More Cup and it's sequel episode. Smart Mort is an intelligent character, while “Groovy 60's cult leader”/Hippie Mort is a peaceful character. It is also revealed that Mort absorbed his own grandmother, and now she lives inside his mind. Also, it is seen in some episodes that Mort has versions of him inside his mind that he can talk to when he needs something. Mort is also portrayed to have some traces of Jewish heritage, as seen in Karl-Mageddon, after his grandfather shows up and a flashback appears of Mort as a child getting a present on Hanukkah from Grandpa Mort. He did mention that he got a pencil.