Roy Clark was the nephew and apparently the only surviving relative of theater producer Walter Clark. In 1947, he and his fellow actor Alice Grant married. When Alice and Roy were cast in Walter's production of "Alice in Wonderland", with Alice playing the role of the title character, the two began plotting to murder Walter so that Roy could inherit his fortune. In order to carry out their elaborate plot, they also sought the aid of the Clark maid Edna who was promise a cut of the money obtained.
Walter soon organized an "Alice in Wonderland" themed party aboard his yacht, inviting many celebrity guests including reporter Betty Dean and the Sub-Mariner. During the party, Roy left the group and changed into a Mad Hatter costume, and began lurking about the ship. He was spotted by Namor but was able to flee. Meanwhile, Alice led the guests to their banquet dinner where a piece of cake was left on the empty table. Alice ate a bite of it, and with the help of a trap door and stilts was made to appear as though she grew in size. She was then rushed to the hospital where she managed to continue her ruse. Roy, as the Mad Hatter, then sent a ransom demand for $10,000 to restore Alice to "normal", which Walter agreed to pay. The Hatter then called Alice telling her to meet him at Lake Park. There, the Hatter took the money and turned over the "antidote" but was then attacked by the Sub-Mariner. Having dressed up his dog in a Jabberwock costume, the Hatter distracted Namor enough to knock out the hero. The Mad Hatter then fled, and Alice removed the apparatus that made her look larger and revived Namor, telling him a false account of what happened.
The Mad Hatter then decided to eliminate the hero, and used Betty as bait. Tricking her to going to his hideout at the abandoned Flamingo Roadhouse, he forced her to call Namor into a trap and gassed both of them. He then left them to die on a chain pulley that would eventually strangle them to death. The Hatter then went to Rock Point to collect a $50,000 ransom from Walter Clark, who was threatened to be "grown" by the Hatter's fake formula. When Clark turned over the money, the Mad Hatter shot him to death and fled the scene just as the Sub-Mariner arrived.
Removing his guise, Roy, Alice and Edna returned to the yacht to make plans to obtain Walter's insurance plan and flee the country when the Sub-Mariner suddenly arrived to investigate the yacht after learning that Roy was to inherit Walter's fortune. Roy attempted to hold Namor at gun point, but the hero managed to distract Roy long enough to knock him out. Roy and his accomplices were then turned over to the police where they made a full confession.
Roy Clark's subsequent fate is unknown.
As the Mad Hatter, Roy used a number of tricks to make it look like he was really the Mad Hatter from "Alice in Wonderland". To imitate the growth potion from the book, he rigged a trap door that pushed stilts onto the legs of Alice Grant, making it appear as though she grew in size. He also developed a costume to disguise his dog as the dragon Jabberwock. This costume somehow allowed the dog to fly.
The Mad Hatter was armed with a pistol.