Kay became an NXT trainee on 13 April 2015. She made her televised in-ring debut on the 10 June episode of NXT, where she competed in a losing effort against Becky Lynch under the ring name Jessie. On 7 August, she was given the new ring name Billie Kay. After competing in most of her matches at NXT as a face, Kay competed on her first match as a heel on the 21 October episode of NXT, losing to Asuka. Through the end of 2015, Kay started being managed by fellow wrestler, Sylvester Lefort, during multiple NXT live events, however it did not last for long after Lefort was released from his contract in February 2016.
On the 13 January 2016 episode of NXT, Kay competed in a number one contender's battle royal for Bayley's NXT Women's Championship, which was won by Carmella. Kay made her first appearance on the main roster on the 30 June episode of SmackDown, where she worked as a jobber losing to Dana Brooke. Kay finally made her return to NXT television on 27 July, where she achieved her first live victory in the company by defeating Santana Garrett. After asking NXT's general manager William Regal for a match at NXT TakeOver: Brooklyn II on the 17 August episode of NXT, she was granted one against the debuting Ember Moon. At the event on 20 August, Kay was defeated by Moon. Following a brief hiatus, Kay returned on the 21 September episode of NXT, where she defeated Aliyah.
In October, Kay started an alliance with Peyton Royce, later dubbed The Iconic Duo, and later entered a feud with Liv Morgan with the duo attacking and defeating Morgan in singles matches. This ultimately led to a six–women tag team match at NXT TakeOver: Toronto, which was taped and aired for the 23 November episode of NXT, in which Aliyah, Ember Moon and Morgan defeated Kay, Royce and their partner Daria Berenato. In the end of December, Kay and Royce were placed in a brief feud with the NXT Women's Champion Asuka after the latter stated there is no competition for her. This resulted in a fatal–four-way match, which also involved Nikki Cross, at the NXT TakeOver: San Antonio event on 28 January 2017, in which both Kay and Royce failed to capture the Women's Championship.
Kay and Royce, now dubbed The IIconics made their main roster debut on the 10 April 2018 episode of SmackDown Live attacking then SmackDown Women's Champion Charlotte Flair, whilst she was cutting a promo about her match at WrestleMania 34. A week later, Kay lost to Flair. In their first match together as part of the main roster, The IIconics racked up their first victory against Asuka and Becky Lynch. Throughout the next few months, Kay competed in various singles and tag team matches but ended up on the losing end. In August, The IIconics started their first feud on the main roster, with Naomi, and the two were able to defeat her in singles matches. Eventually, Naomi teamed up with Asuka but lost to the IIconics at WWE Super Show-Down on 6 October, held in the latter's homeland of Australia. Three weeks later, both Kay and Royce took part in WWE's first all-women's pay–per–view, Evolution; they were the first two eliminated from a battle royal for a future women's championship match.
On 27 January 2019, both Kay and Royce entered their first Royal Rumble match at number 7 and number 9 respectively, and they managed to eliminate Nikki Cross, before they both were eliminated by Lacey Evans. On February 17, at the Elimination Chamber, The IIconics competed in a tag team Elimination Chamber match for the inaugural WWE Women's Tag Team Championship, which was won by Sasha Banks and Bayley. In March, The IIconics started a feud with Banks and Bayley, whom they defeated in a non–title match. Because of their win, they (and two other teams) challenged Banks and Bayley for the championship at WrestleMania 35 in a fatal four–way match. At the event, which took place on 7 April, The IIconics won the match after Kay pinned Bayley to win the Women's Tag Team Championship for the first time. On the August 5 episode of Raw, The IIconics lost the titles to Alexa Bliss and Nikki Cross in a fatal-4-way match. On 16 October, it was announced that The IIconics had been drafted to Raw as supplemental picks of the 2019 WWE Draft.
After a brief hiatus, Kay and Royce made their return on the May 11, 2020 episode of Raw, interrupting WWE Women's Tag Team Champions Alexa Bliss and Nikki Cross. They later defeated the champions in a non-title match. They would unsuccessfully challenge for the WWE Women's Tag Team Championships multiple times throughout the summer. They would then begin a feud with Ruby Riott, mocking her backstage for not having any friends. They would go on to trade victories as Kay and Royce defeated Riott while Riott defeated Kay. At Payback, The IIconics were defeated by Riott and her newly reunited tag partner Liv Morgan. The following night on Raw, The IIconics were forced to disband after losing to The Riott Squad per stipulation.
As part of the 2020 draft in October, Kay was drafted to the SmackDown brand. Kay would then begin a storyline on SmackDown where she would offer superstars such as the Street Profits and Big E to look at her resume, only to be rejected by them. On the January 1, 2021 episode of SmackDown, she helped The Riott Squad defeat Natalya and Tamina much to their dismay. Over the next few weeks, Kay would try to persuade The Riott Squad to add her to their group as she accompanied them to the ring during their matches and attempted to interfere in matches on their behalf, seemingly turning face for the first time since October 2015. However, Kay resumed as a heel when she teamed with Carmella for a tag team turmoil match at WrestleMania 37, on night one, which was won by Natalya and Tamina. Billie Kay (along with her former partner Peyton Royce) was released from WWE on April 15, 2021.