Max is Grandfather to Ben and Gwen. He has taken the kids on a summer road trip across the country. It is during this time that Ben finds the Omnitrix. Max helps Ben and Gwen come into their powers while trying to hid the fact that he fought Aliens, instead of being a real Plumber. In an early episode one of Max's old partners, turned up and was fighting aliens. Many of the aliens Max (grandpa) started recognizing as ones they had previously captured and placed in the Null Void Projector. As the episode progressed it was revealed (shocker) that Grandpa’s old partner Phil was using the Null Void Projector to release the aliens so that he could fight them again to regain his importance. He had hoped Max would join him, but Max choose to side with his family.
Throughout the episodes Max has shown his worth with both his intelligence and his abilities to their “team” of alien fighters.
In a made for T.V. live action movie Max Tennyson was played by Lee Majors.
In Ben 10 Alien Force, Max Tennyson is shown to be asking for Ben's help on a case. It has been 5 years since Ben, Max, and Gwen have had any adventures. So when Ben was still at the age of 10, he decided to remove the Omnitrix. Locking it away in his cabinet. However, Ben donned the Omnitrix once more and got a new set of aliens. Max is not seen a lot in this series, he got a physical appearance in episode 6 called "Max Out." Where he teleported himself and a Hybrid to the Null Void. Ben and his team, thought that he was killed.