Millicent was the daughter of respected explorer Montgomery Clyde, who was on an exploration quest on the darkest regions of Peru. There, he found a new species of bears, highly intelligent and friendly ones. He befriended them after one called Pastuzo saved his life, leaving them plenty of things as a gift. However, his society refused to accept his discovery as he had not brought back a dead specimen to prove it. Though he was given a chance to get a specimen, Clyde refuses to let anyone else go there and kill his new friends, so he was kicked out of the society and forced to open a petting zoo to make a living. Millicent was only a little girl when she witnessed all of this and felt bitter towards her father for ruining his chance of becoming the most greatest man in the world, so she vowed to succeed in what her father failed to do.
Years later as a grown woman following the death of her father, she had a position at the British Museum as head of the Taxidermy Department; however, not content to watch over old-stuffed animals, she had living animals illegally imported to add to the collection. Upon learning that one of the intelligent bears named Paddington has arrived to London to find himself a new home, Millicent senses an opportunity and vows to hunt Paddington down and stuff him for the museum in order to restore her father's former glory for herself.
Upon learning that Paddington is temporarily residing with the Brown family, Millicent conspires with their neighbor Mr. Curry to capture Paddington. The first attempt was a failure, though this result half of the Brown's house to be burned. Upon seeing that the Browns won't believe his side of the story, Paddington leaves to find Montgomery Clyde himself. When he already locates the explorer's house, he soon learns of the explorer's death and that Millicent was his daughter. Expecting his arrival, Millicent tranquilizes Paddington and brings him to the museum, and prepares to have him stuffed.
However, Mr. Curry, having a change of heart after realizing her true intentions, informs the Brown family of the events, and they manage to rescue Paddington from being stuffed. However, they are cornered on the roof by Millicent, who threatens to tranquilize and stuff them as well if they refuse to hand over Paddington, saying that she has no qualms of stuffing her own kind. Playing wisely on this, Paddington subdues Millicent by throwing a marmalade sandwich at her, which attracts a huge flock of pigeons to distract Millicent, allowing the Browns' maid Mrs. Bird to open a roof hatch, inadvertently sending Millicent to fall off the roof and dangle on a flagpole, much to her dismay. The Browns then decide to accept Paddington as a new member of their family, much to his delight.
It is later revealed in the end that Millicent is arrested for her crimes and sentenced to community service at a petting zoo (presumably the same one that her father founded), much to her complete horror (as she begged for a different punishment to no avail). She is last seen scooping donkey manure before being buried in a pile of mulch by the same monkey that she tried to stuff earlier, much to her anger.