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:
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 episode of NXT, Daddio made her return under the new ring name 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 episode of NXT, she competed in a six-woman tag team match, along with Carmella, and Nikki Cross, in which they defeated Daria Berenato, Mandy Rose and Alexa Bliss.
On the August 31 episode of NXT, Morgan picked up her first televised singles win, defeating Aliyah. On the September 14 episode of NXT, Morgan defeated Rachel Fazio in singles competition via submission, and would issue a challenge to the NXT Women's Champion Asuka. The following week, however, Morgan was submitted by Asuka in under a minute, and would afterwards begin a rivalry with The Iconic Duo (Billie Kay and Peyton Royce), with both women confronting her following her match, leading to match between Morgan and Kay two weeks later, with Kay picking up the victory after interference by Royce. On the October 26 episode of NXT, she interrupted a match between Kay and Aliyah by attacking Royce, causing a distraction that allowed Aliyah to defeat Kay. Morgan went on to compete against Royce on the November 16 episode of NXT, which ended in a disqualification after Kay interfered in the match, attacking Morgan and Aliyah until Ember Moon made the save. The three faced off in a six-person tag team match against Kay, Royce, and Daria Berenato on the November 23 episode of NXT, with Morgan's team coming out victorious.