Millicent's first night with her relatives seemed uneventful to her, and she complained much too often about the Collins estate. She was particularly disturbed about Jeremiah Collins’ attitude toward her, and believed he was trying to avoid her. Millicent, along with the rest of the family, was stunned when Jeremiah suddenly married Josette, Barnabas’ betrothed; they were under a spell. Millicent broke into tears after finding out, and selfishly whined to the other family about her situation. Millicent's ways changed somewhat when Jeremiah died following complications from a duel wound. She nearly fainted when she saw his body. She later attend her cousin's funeral and was present when the fanatical witch hunter Reverend Trask and Millicent's equally fanatical cousin, Abigail Collins had the family's new governess Victoria Winters arrested on a series of charges of witchcraft.
Millicent became the guardian of the Collins children Daniel and Sarah during the following horrific nights at Collinwood. She shielded them from the knowledge that the witch Angelique had really murdered Jeremiah - subsequently making him her zombie slave - and Barnabas before dying herself. However, responsibilities did not change her attitude toward people of lesser rank than she was. She continued to berate the servants of Collinwood, especially Ben Loomis. When Joshua Collins announced that his family would be moving to the newly built Collinwood Millicent wished to transfer her luggage from the Old House to Collinwood, but was told by Ben that not all of her belongings could be brought over in a single night. She employed Ben the following evening to help load the luggage into a carriage.
While Ben loaded luggage, Millicent began arranging her packages and trunks. Busy, she glimpsed a shadowy figure from across the manor hall, mistaking it for Ben. It was in fact Barnabas - newly raised as a vampire by Angelique's curse. Millicent screamed, trying to fight Barnabas off but unwittingly became her vampire cousin's first victim. Ben discovered her fainted and bleeding profusely on the hall floor. She was rushed to Collinwood immediately, where the family treated her loss of blood. While unconscious, Millicent deliriously echoed Barnabas' name. Joshua had Millicent's attack concealed as fever. Despite the family's best efforts, Millicent's condition worsened. That night, as Barnabas prepared to attack his sister-in-law Josette, Millicent stopped him from doing so by offering herself to him instead. The next day, the children discovered her dead body in the woods. She was interred in the Collins Mausoleum that afternoon. Joshua had Millicent's death doctored in the Collins Family History Book, claiming she died of her "fever".
As there was no hint in the rest of the series that Millicent returned as a vampire (despite dying as a result of Barnabas' bite), it is likely that Joshua ensured she was staked to prevent her resurrection after learning of his son's continuing existence. Alternatively, she died as a result of shock related to blood loss, and as such did not become a vampire (much like Gloria and her boyfriend did in 1990).