However, he did design the very first set of cards for the game, which was released in 1999 and featured a specific set of monsters, spells, and traps.
It's difficult to say definitively which card was the *absolute* first, as it was part of a cohesive set. But you could argue that some of the key cards from that initial set, like "Dark Magician" or "Summoned Skull", represent the earliest creations within the Yu-Gi-Oh! TCG.