Mopey made only one appearance in the series, during the episode "Prime Mates". Mopey returned to the lair of Mojo Jojo seeking Mojo's acceptance. Mojo did not particularly care for Mopey, but agreed to let him help with the destruction of the Powerpuff Girls after he got the impression that Mopey Popo was going to kill him (he was really just reaching for some banana jerky).
Mopey soon proved to be a terrible assistant, constantly breaking things and messing up experiments. Mojo soon threw him out, whereupon he swiftly returned with the Powerpuff Girls tied up and unconscious. It is never explained how he managed this.
Shortly after returning to Mojo with the Girls, Mopey leaves, saying that taking over the world isn't all it's cracked up to be. He also releases the Powerpuff Girls, thinking that they were Mojo's friends, whereupon they of course beat the stuffing out of Mojo after seeing him begging Mopey to tell him how he was able to capture the Girls in the first place. He is voiced by Dee Bradley Baker.
He is the only one of Mojo's former army to appear in the series after the film.