The Mamba du Mal is a snake that was once a part of Dr. Montgomery Montgomery's collection. The Mamba du Mal is "one of the deadliest snakes in the hemisphere," and kills its victims by strangling them, leaving dark purple bruises. Count Olaf injected its venom into Uncle Monty, which killed him, but because Uncle Monty did not have any bruises, Klaus was able to prove that the Mamba du Mal was not responsible for his death. The snake is also capable of communicating, and was trained by V.F.D. to communicate vital messages as evidenced by the following excerpt from The Mamba du Mal: A Snake That Will Never Kill Me by Tony "Mommy" Eggmonteror: "More pleasant to contemplate, however, are the snake's excellent communication skills. Certain specimens from the Mamba du Mal have been trained to recite certain phrases in an encoded form of English so they might be employed as guardians of crucial headquarters (VFD). A Mamba du Mal hissing the phrase, "Summer is," for instance, is communicating a coded version of the phrase, "Enemies are nearby." The hissed phrase, "Over and gone," translates to, "probably in disguise," and the Mamba has been known to hiss the word "dying" as a code word for "Beware of arson." The only other creature with communication skills sufficient to convey these messages is the common grass cricket."
At the end of The Reptile Room, the snake is packed up with the rest of Monty's collection by the Herpetological Society. Later, in Lemony Snicket: The Unauthorized Autobiography, the Mamba du Mal is killed by unknown means, and its body (which is now framed) is auctioned off to the "Esmé Squalor Fan Club" at the In Auction.
- The Mamba du Mal never appeared in the 2004 film, Lemony Snicket's A Series of Unfortunate Events. Its role of the accused murderer of Dr. Montgomery was instead played by the Incredibly Deadly Viper. It is possible that it is in Uncle Monty's house somewhere amid all the reptiles.