Carl Tennyson (voiced by Dee Bradley Baker in the first series and played by Don McManus in the live-action film and the second series) is Max's son, Sandra's husband, Ben's father, Frank's brother, and Ken and Gwen's uncle.
In the live-action film "Ben 10: Race Against Time", he is depicted as a new-age style parent who wants Ben to have his "freedom," insisting on being called "Carl", not "daddy", "dad" or "father", and letting Ben run off on mysterious adventures without asking questions. Carl wants his wife to give their son some freedom. He seems to be even more laid back than his wife, and is seen at least twice telling her to let Ben have his fun. For instance, near the end when Ben's mother tells him to not eat too much cheese, Carl says to "Let the kid live a little. Eat all the cheese you want, Ben!" In his first animated appearance, "Goodbye and Good Riddance", he is shown to resent his father, as his work as a plumber (the sanitary kind) prevented him from spending time with his family. He finds out about the Plumbers and the Omnitrix the hard way when Vilgax destroys their house and holds him hostage, leading to Ben as Cannonbolt saving him. He later assists Ben, Gwen, and Max in defeating Vilgax and saving Bellwood, and he and Max reconcile somewhat.
Carl later appears in the Ben 10: Alien Force episode "Grounded", where he and Sandra discover that Ben has resumed his superhero activities alongside Gwen and Kevin. (Though it appears this was the first time they discovered this, thus making "Goodbye and Good Riddance" non-canon.) Apparently unable to handle the thought of their son constantly risking his life, and angered about Ben's constant lying about his activities, they ground him, forbid him from using the Omnitrix, and tell Gwen's parents of her superheroism, resulting in her grounding as well and Kevin being forced to fight the Highbreed alone. Additionally, Carl was furious when he found out his father had something to do with it. When this proves to be too much for Kevin to handle, Ben openly defies his parents and uses the Omnitrix to (literally) break out of the house and help Kevin; through this, they realize just how important Ben is to the world. They later come to help Ben and Kevin fight, and admit that they are proud of Ben, openly allowing him to resume his superheroism. In this episode, he is modeled after and voiced by the actor who played his live-action counterpart in the film.