- “Your grandfather had a saying: Our lives are remembered by the gifts we leave our children.”
- ―Preston B. Whitmore
Thaddeus is mentioned by Milo that he acted more like a father to him after his parents' death. He and Milo would often go out camping and tell stories about the lost continent Atlantis. He is also very stubborn, as he kept tempting Whitmore with finding the Shepherd's Journal. Though in his final year, he died heartbroken after years of being the laughing stock by the board members at the museum.
It is known that Thaddeus has been friend with Whitmore for a long time, the two of them having known one another since their university days in 1860s, and when Whitmore got fed up about the Shepherd’s journal existence, they enter into a bet where if Thaddeus finds the Shepherd's Journal, Whitmore will fund the expedition and even kiss him full on the mouth. In 1911, Whitmore funds the expedition that finds the book in Iceland and when he came back it became an embarrassment of the both of them. But shortly afterward, Thaddeus passes away. According to Whitmore, like Milo, the board members made fun of him. Prior to his death, Thaddeus would leave the Shepherd's Journal in the possession of his long-time friend Whitmore, with the instruction that he would bring the gift on to Milo when he was ready. Thaddeus did not have many friends at the museum, but it is known that he was a close friend of Joshua Sweet, with whom he travelled to Iceland, and told him a phrase 'when you've hit the bottom, the only place left to go is up'.
Though Thaddeus unfortunately never got to accompany the expedition that would eventually discover the lost city of Atlantis, he remained with Milo in spirit, providing an important source of motivation for his grandson.
A comic book prequel story printed as a Disney Festival Special in the Netherlands set before the Iceland expedition, the story opens in Washington, DC, with Thaddeus Thatch reporting his findings of the Journal's supposed whereabouts to Preston Whitmore and making the bet. When research in the Library of Congress results in believing his findings to hold water, he is tailed by thugs that capture him and ransack his home to seize his research, knowing that a job calling for the services of Rourke must be for a incredibly valuable target. After being rescued by the team, he reveals he managed to hide his journal under a statue of Lincoln in a park. When the kidnappers catch up to them, they manage to seize Thatch's journal after holding a member of the team at gunpoint, but he managed to tear out the final page with the location of the Shepherd's Journal during the confusion, allowing the Iceland expedition to continue.