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, Carolyn 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 Carolyn had a daughter together named Amy Preston. Wallace got Carolyn to start smoking which caused her to become gravely ill.
In the altered timeline, Lucy, Wyatt Logan, and Rufus Carlin prevent all deaths on the Hindenburg. Because of this, Henry Wallace marries Elizabeth Merrick, granddaughter of a Hindenburg survivor Irene Doehner. In this timeline, Wallace and Carolyn never meet, resulting in Amy Preston never being born, Carolyn becoming well because she never started smoking, and Lucy finding out that Henry Wallace was never her biological father. (“Pilot”)
Carolyn still met Benjamin Cahill while she was a junior in college and he was a professor and had a relationship with him. 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.
Later, Carolyn reveals to Lucy that her biological father is Benjamin Cahill. Carolyn later also reveals that not only is Benjamin a Rittenhouse member, but so is she. (“The Red Scare”)
Carolyn wanted to shield Lucy from the reality of Rittenhouse when she was young, as she herself thought she learned about it too soon. Nevertheless, Carolyn wanted to introduce Lucy to Rittenhouse at some point, have her join it, and continue the family legacy, but Lucy has continued to resist her and Rittenhouse. Due to the fact that she can't remain objective about Lucy being a threat and not a potential recruit, Nicholas Keynes has taken away her authority over Lucy, and has focused efforts on killing Lucy instead of inducting her. To try to protect Lucy, Carolyn captured agent Denise Christopher and threatened the life of her wife, Michelle Christopher, to make it so Lucy is not put on time travelling missions anymore.
After protecting Lucy from Emma's gunshot on a mission in 1888, Emma got tired of Carolyn's allegiance to Lucy. Emma proceeded to shoot Carolyn, and then killed Nicholas Keynes when he reacted. Lucy attempted in vain to provide a doctor for Carolyn. As Carolyn died, she expressed her only regret, that being not introducing Lucy to Rittenhouse sooner, so that she could accept the benefits it could have offered her.