Daddio was born in Paramus, New Jersey. She grew up in Elmwood Park, New Jersey and attended Henry P. Becton Regional High School in East Rutherford,NJ. She has four older brothers and a sister, and following the death of her father, Daddio's mother was a single parent raising six children. Daddio is a longtime fan of professional wrestling, and would participate in backyard wrestling with her siblings. Daddio described the matches as core to start her foray into wrestling, saying:
"I would literally take pedigrees and powerbombs like it was my job. I always pretended to be Lita. All the Divas were so glamorous and girly, and I was such a tomboy and didn't have nice clothes. When I saw Lita come out in baggy pants and sneakers, wrestling the boys, I thought she was the coolest thing ever. I related to her."
In 2014, after being discovered at the Joe DeFranco's gym in Wyckoff, New Jersey, Daddio signed a contract with WWE, assigning her to their developmental territory, NXT in October 2014. She made her first televised appearance at the NXT TakeOver: Rival event on February 11, 2015, as a planted fan, who jumped on Tyler Breeze during his ring entrance. She later appeared at NXT TakeOver: Unstoppable on May 20, again as part of Breeze's entrance. In October 2015, she briefly worked under the ring name Marley, debuting on the November 4 episode of NXT, where she worked as a jobber losing to Eva Marie.
On the December 2, 2015 episode of NXT, Daddio made her return under the new moniker Liv Morgan, losing to Emma. On the January 13, 2016 episode of NXT, Morgan competed in a battle royal to determine the number one contender to Bayley's NXT Women's Championship, which was won by Carmella. On the August 17, 2016 episode of NXT, she competed in a six-woman tag team match, along with Carmella, and Nikki Glencross, in which they defeated Daria Berenato, Mandy Rose and Alexa Bliss.