Mr. Mysterio is a hypnotist who dresses up like the ghost pirate Wooden Leg Wally and the first mate of Captain Skunkbeard.
Mr. Mysterio is first seen wearing his Wooden Leg Wally costume when he and billionaire Biff Wellington (in his Captain Skunkbeard costume) and their henchmen invaded (and supposedly sunk) Rupert Garica's boat. Later the following morning, when Scooby-Doo and his friends arrive to go on a cruise to the Bermuda Triangle with Fred Jones's parents, Skip and Peggy Jones, in honor of Fred's birthday, Mysterio pops out of his Wooden Leg Wally costume and scares Scooby. Once on the cruise, he scares Scooby again, along with Shaggy Rogers.
Later, while Scooby and Shaggy are hunting an Alien, he scares them again. After Scooby and Shaggy had re-unmasked the Ghost of Captain Cutler, Mysterio scares them again. Later at the dinner, the hypnotist scares them one more time and tries to hypnotize them but instead hypnotizes Fred's mom and the other people in audience, minus Fred, Fred's dad, Rupert (who survived the shipwreck and has swapped clothes), Velma Dinkley and Daphne Blake. He then disappears. Later he reappears wearing his Wooden Leg Wally costume with and Wellington (in his Captain Skunkbeard costume) and their henchmen. They capture some of the guest, even Fred's parents, promoting the gang to rescue them.
While holding Fred's dad hostage (thinking he's Rupert), Mysterio/Wally is anger that he is unable to get him to "Spill the fish" about directions to the Bermuda Triangle gold meteor called the Heaven's Light. When Skip says "I keep telling you!" Mysterio/Wally orders Skip to belay his jabbering. Later after Scooby and the gang, along with Rupert make abroad the pirate ship to rescue the captives, they capture all the pirates, except Mysterio/Wally. After unmasking Skunkbeard and revealing that he is Wellington and forcing him to explain his evil plan. Wally removes his mask and reveal that he is Mr. Mysterio.
Mr. Mysterio reveals that he convinced Wellington that he was the ancestor of a famous pirate. He wanted the meteor because it was solid gold which made him richer than Wellington. Scooby decides to drop the meteor back back in (because it was already pulled out), Mysterio tried to prevent the dog from doing so, however Scooby tickles Mysterio and drops the meteor back in the triangle. The gang later discovers that the other pirates are the other captives. Velma realizes Mysterio hypnotized them to be pirates in his plan. Even Fred's mom was hypnotized to be a pirate. Velma demands Mysterio release his victims from his spell. The hypnotist refuses, but when Scooby threatens to tickle him again, he releases them form his spell. Mysterio and Wellington are later taken out of their pirates and last seen tied up together.
- He was voiced by Dan Castellaneta both in and out of the Wooden Leg Wally costume.