Eli is the main antagonist of Jurassic World Fallen Kingdom. He was the assistant and finance manager of Benjamin Lockwood, one of the co-founders of the original Jurassic Park (alongside John Hammond). He seeks to make a profit off of selling the dinosaurs trapped on the island of Isla Nublar by any means necessary, which includes killing those who either outlived their usefulness or stand in his way.
Mills first appeared in Lockwood's estate, where he greeted Claire Dearing (the former operations manager of Jurassic World) and told her about the impending crisis of Isla Nublar, as an imminent volcanic eruption may destroy the island and kill all of the dinosaurs residing there, and that the government will not rescue them because doing so would lead to disastrous results for the mainland. Mills told Claire that Lockwood has financed an operation that will help rescue the dinosaurs from the cataclysm and transport them to another island as their new reserve. He also tells Claire to hire Owen Grady for the mission due to his extensive training of Velociraptors (particularly the last surviving one known as Blue).
Accompanying Owen and Claire were Ken Wheatley and his team of mercenaries, whom Mills had hired to capture the dinosaurs for him to make money. After capturing Blue, Wheatley and his mercenaries betrayed Owen, Claire and their friends by leaving them on the island to die while capturing more dinosaurs and bringing them to the ship in cages. This made both a distraught Owen and Claire realize that the rescue operation was nothing more than a ruse to capture the dinosaurs and sell them off for profit. It also reveals that Mills previously sent a mercenary team to recover a specimen of the deceased Indominus rex in order for Henry Wu to create a new deadly hybrid dinosaur for weaponry: the Indoraptor.
Though Owen, Claire and their friends manage to evacuate the exploding island, they watch sadly as an unfortunate Brachiosaurus is left to die in the inferno of the island. Once the group arrives at Lockwood's Mansion, they end up being apprehended by Wheatley and locked up by Mills, who reveals that he's behind the plot and that he intends to sell the dinosaurs including the Indoraptor as weapons to arms dealers and warlords for profit (just like Vic Hoskins), all while mockingly calling Owen and Claire the "Parents of the New World". When Lockwood finds out about Mills' true intentions from his granddaughter Maisie, he confronts Mills for his betrayal, but Mills murders Lockwood by smothering him with a pillow while confining Maisie to her room.
Just as Mills proceeds with his plan by hiring an auctioneer Gunnar Eversol to auction off the captured dinosaurs, Owen manages to release himself and Claire by having a Stygimoloch to break them out and disrupt the auction, just as the Indoraptor is being presented for auction against Henry Wu's objections. Mills, horrified that Owen and Claire escaped and ruined the auction, tried to tell his men to retract the track so the buyer could get the Indoraptor, but Owen foiled it and disconnected the track to prevent the auction from proceeding further. After the Indoraptor escapes and kills Wheatley and Eversol, Owen rescues Maisie before he and Claire are confronted by an enraged Mills, who reveals a shocking truth: Maisie is actually the clone of Lockwood's real daughter and the true cause why Lockwood and his partner John Hammond broke up (as Hammond opposes human cloning). Just then, the Indoraptor kills the two mercenaries and attacks the group, allowing Mills to escape with the Indominus sample. Fortunately, Owen gets Blue to attack the Indoraptor, sending the hybrid to fall through a glass roof and getting impaled on the spiked horns of a Agujaceratops skull on display.
Maisie then freed the other dinosaurs from captivity into the mainland, much to Mills' distraught as he sees his remaining men being trampled to death by the dinosaurs. Mills tries to take the Indominus sample to escape, but by an unexpected surprise, the park's original Tyrannosaurus, Rexy grabbed him with her jaws and devoured him before crushing the Indominus rex sample, destroying the last piece of evidence of the I. rex's existence. All that's left of Mills was his right leg, which was detached when Rexy devoured him before she escaped with the other dinosaurs.