The Joker, one of Batman's most iconic adversaries in DC Comics, first appeared in Batman #1 in the spring of 1940 as a homicidal criminal mastermind with a distinctive clown-like appearance and a penchant for elaborate, joke-themed crimes. In the 1966 Batman television series, the character was adapted into a campy, prank-obsessed villain portrayed by Cesar Romero, who made his debut on January 26, 1966, in the two-part episode "The Joker Is Wild" and "Batman Is Riled" (Season 1, Episodes 5 and 6). This marked the first live-action depiction of the Joker, emphasizing his chaotic humor and rivalry with the Dynamic Duo while toning down the darker elements of his comic book origins to suit the show's lighthearted, family-friendly tone.

Cesar Romero's performance brought boundless energy to the role, infusing the Joker with a manic cackle, theatrical flair, and an array of gimmicky gadgets such as joy buzzers, acid-squirting flowers, and exploding cigars. Notably, Romero refused to shave his trademark mustache for the part, resulting in it being faintly visible beneath the white makeup—a detail that became a signature quirk of this incarnation. The character's schemes often involved absurd yet ingenious plots, such as tampering with school vending machines to corrupt youth in "The Joker Goes to School" or entering art contests in "Pop Goes the Joker," highlighting his intelligence, creativity, and utter amorality.

Throughout the series' run from 1966 to 1968, the Joker appeared in nineteen episodes, frequently clashing with Batman and Robin through escalating pranks and traps, and occasionally allying with other villains in the 1966 feature film Batman. Subtle developments included hints at a pre-criminal past as a hypnotist, revealed in "The Impractical Joker," adding layers to his persona beyond mere clownish villainy. Romero's portrayal established the Joker as a charismatic Clown Prince of Crime, influencing subsequent interpretations while capturing the era's playful exaggeration of comic book tropes.