In the original timeline, Carolyn Preston met Henry Wallace in 1979 while at the University of California, Berkeley. The two said that it was love at first sight, and later married. However, Carol eventually had an affair with Benjamin Cahill that resulted in the birth of Lucy Preston in 1983. Lucy was raised believing Henry was her father. In 1990, Henry and Carol had a daughter together named Amy Preston. Henry got Carol to start smoking, and she later contracted terminal lung cancer.
After the Hindenburg disaster is prevented by Garcia Flynn, the timeline is altered. Henry and Carol never meet because Henry marries the granddaughter of a Hindenburg survivor. The butterfly effect results in Amy never being born, Carol becoming healthy, and Lucy finding out that Henry Wallace is not her biological father. (“Pilot”)
Carol still had a relationship with Benjamin Cahill while she was a junior in college and he was a professor. In the altered timeline, however, he offered to marry her after she told him she was pregnant and she refused, after which they never spoke again. (“The Alamo”)
When Lucy later tells her mother about her time travelling and her plan to get Amy back, Carol reveals she is a Rittenhouse member, just like Cahill. (“The Red Scare”) She thought she convinced Lucy to join Rittenhouse and brought her back to 1918 so they could rescue Nicholas Keynes. However, Emma Whitmore revealed Lucy was planning to kill them, and was ordered to kill her if she interfered in the mission. Instead, Lucy was rescued by Wyatt Logan and Rufus Carlin. Lucy gave Carol the opportunity to join them, but Carol left with Emma and Nicholas. Back in 2018, she revealed to Nicholas that she is his granddaughter. (“The War to End All Wars”)
Carol clashed with Emma over her protecting Lucy, and eventually was ordered by Nicholas to kill Lucy. Carol still tried to get Lucy to side with Rittenhouse and join them, but Lucy refused, saying she'd rather be killed. After Carol failed, Nicholas assigned Emma to kill Lucy, which Carol wasn't happy with. (“The Salem Witch Hunt”)
To try to protect Lucy, Carol captured Agent Denise Christopher and threatened the life of her wife, Michelle Christopher, to take Lucy off the time travelling missions. Carol revealed she wanted to shield Lucy from the reality of Rittenhouse when she was young, as she herself thought she learned about it too soon. However, Lucy refused to back down from the missions and was determined to stop Rittenhouse. (“The Kennedy Curse”)
After protecting Lucy from Emma's gunshot in 1888, Emma got tired of Carol's interference, and proceeded to shoot Carol, then kill Nicholas when he reacted. Lucy attempted in vain to provide a doctor for Carol. As Carol was dying, she expressed her only regret was not introducing Lucy to Rittenhouse sooner, so she could accept the benefits it could have offered her, which disgusted Lucy. (“Chinatown”)