Preston Whitmore is an eccentric millionaire, industrialist, and philanthropist. He attended Georgetown University where he met and befriended Thaddeus Thatch, maintaining a lasting friendship until the latter's passing. Both would graduate in 1866 with Thatch becoming an archeologist and Whitmore beginning his rise to financial success with Whitmore Industries.
While Whitmore was regarded as a visionary pioneer through his company's advancement in technological progress, and despite his eccentric personality, Whitmore was an extremely private person. He only made public appearances when necessary. Otherwise, he was reclusive and only allowed those closest to him to meet with him.
He was also a business rival of Eric Hellstrom, who ran a shipping empire; this rival became known as the Modern Odin once he bought a castle in the old country and named it Asgard. After the Stock Market crash, Whitmore lost contact with Hellstrom for many years; he believed his old rival would go insane.
The filmmakers felt there needed to be a character in the film to act as the financier to fund the expedition. The character was further developed to have been a friend of Milo's grandfather so that he would have a personal investment into the discovery of Atlantis. Whitmore was originally to have been voiced by Lloyd Bridges and he managed to record a few lines. However, he did not record enough dialogue before he passed away of natural causes on March 10, 1998.
In recasting the voice of the character, the filmmakers came upon John Mahoney. John's performance would redefine the character as he brought zest and vigor. Redone as a more eccentric character, this also further enhanced his relationship with Milo's grandfather, which would allow for audiences to truly believe the two were good friends. This also makes Whitmore, as a character, similar in appearance and personality to John Hammond from Jurassic Park. Mahoney enjoyed voicing the character, noting that voice acting in animation allowed him to be big and outrageous, resulting in the character being fond of yoga. One interesting challenge for supervising animator Shawn Keller was having to two characters who, while both were eccentric, were still different in personality.
With Whitmore having been redesigned personality-wise, this also led to the character being redesigned visually. As opposed to being sage-like, the eccentric personality meant they needed an animator to match the new character development. After a while, the filmmakers decided to pick Keller, who was already the supervising animator of Cookie Farnsworth and because the characters had seemingly matching personalities, it seemed natural to use the same animator for the two.
Whitmore and Thaddeus 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. Much to his simultaneous astonishment and embarrassment, Thatch is able to find the journal. Shortly afterward, Thaddeus passes away.
Three years later, needing a linguist to translate the directions to finding Atlantis, Whitmore has his personal assistant Helga Sinclair retrieve Thaddeus' grandson, Milo James Thatch. He convinces Milo to participate in the expedition in place of his grandfather and even provided everything else such as approval Milo's resignation from the Smithsonian Institute, packing his belongings with his books in storage, and even taking care of his cat Fluffy while he is away.
Whitmore stays behind during the expedition. When only a handful returns, they inform him of what has happened. Understanding of situation, he briefs everyone into the secrecy of the expedition and to rehearse a false story should anyone ask. Whitmore goes over pictures that Wilhelmina Packard took when he comes upon a package addressed to him. Inside is a picture of Milo as a child with Thaddeus, a crystal, and a message thanking him on behalf of the Thatches.
Thanks to the crystal, Whitmore would feel 20 years younger wearing it. "Though that still leaves me pretty old."
Whitmore would join the others in venturing to Atlantis briefly to inform Milo and Queen Kidagakash of what is happening on the surface. Once the adventure in Krakenstaff was over, he had the group over for dinner at his mansion, offering Milo and Kida the chance to see the world; he would pay for everything. However, Doc Sweet's radioing Packard for help ruined the chance.
During this adventure, an old spear was taken from his home. He called the group together and told them about Eric Helstrom, who he believed was now completely insane; he called previously, begging him to sell the spear while raving about Ragnorok. He sent the group to Norway to stop Helstrom before he could end the world.
Once everything had settled down, he joined everyone in Atlantis again and watched as Kida had the city raised above the ocean waves once more. His last lines of the movie suggested the world was better thanks to Atlantis' return to the surface.