Harper vs. Bell, Obenauf

Following a stoppage win against Claudia Lopez in May,^[16] Harper faced former world title challenger Nozipho Bell on 19 July for the vacant IBO female super-featherweight title at the Magna Centre in Rotherham. Harper dropped her opponent twice in the eighth round before referee Howard Foster called a halt to the contest, handing Harper her first world title via eighth-round TKO.^[17][18]

It was announced in September 2019 that Harper had signed a promotional contract with Eddie Hearn's Matchroom Boxing, with her first fight under the new promotion to take place on 2 November against former two-time world title challenger Viviane Obenauf at the Manchester Arena. The fight was televised live on Sky Sports as part of the undercard of Katie Taylor's world title bout against Christina Linardatou.^[19] Harper successfully retained her IBO title via unanimous decision (UD) over ten rounds, with the judges' scorecards reading 99–91, 99–92, and 97–93. In the post-fight interview, Hearn announced that terms had been agreed for Harper to challenge reigning champion Eva Wahlström for the WBC female super-featherweight title in early 2020.^[20][21]

Harper vs. Wahlström

On 20 December it was announced that Harper would challenge Wahlström on 8 February 2020, at the FlyDSA Arena (formerly Sheffield Arena), with Harper's IBO and Wahlström's WBC titles on the line.^[22] The fight was aired live on Sky Sports in the UK and DAZN in the US as part of the undercard for Kell Brook vs. Mark DeLuca.^[23] Harper knocked Wahlström down in round seven en-route to a UD victory, adding the WBC to her IBO title to become the second British female boxer, after Nicola Adams, to capture a major world title. Two judges scored the bout 99–90 and the third judge scored it 98–91.^[24]

Harper vs. Jonas

She defended her titles against former Olympian Natasha Jonas on 7 August 2020, at the Matchroom Sport headquarters in Brentwood, Essex. After a closely contested fight which saw Jonas suffer a cut above her right eye in the early rounds before staggering Harper in the eighth, the result went to the judges' scorecards; one judge scored the bout 96–94 in favour of Harper, the second judge scored it 96–95 to Jonas, while the third judge scored it even at 95–95, resulting in a split draw to see Harper retain her titles.^[25] The bout was the first time two British women contested a world title.^[26]

Harper vs. Thanderz

In the second defence of her WBC title and fourth of her IBO, Harper faced former European female super-featherweight champion Katharina Thanderz on 14 November 2020. The bout served as part of the undercard for Katie Taylor's world title defence against Miriam Gutiérrez at The SSE Arena in London. Thanderz suffered an injury to her nose after an accidental clash of heads in the ninth round before being hurt with a punch to the body, causing her to bend over in pain. Harper followed up with a sustained assault, landing combinations to the head and body, prompting referee Victor Loughlin to call a halt to the contest to award Harper a ninth-round TKO victory.^[27][28]

Harper vs. Choi

In April 2021, it was announced that Harper would face WBA female super-featherweight champion Hyun Mi Choi on 15 May, at the AO Arena (formerly known as Manchester Arena). The bout was to be televised live on Sky Sports in the UK and streamed live on DAZN in the US and more than 200 countries and territories.^[29] In early May, Harper was forced to withdraw from the bout after suffering a hand injury during sparring.^[30]

Harper vs. Ryan

On 23 March 2024 in Sheffield, England, Harper challenged Sandy Ryan for her WBO welterweight title, but lost the fight via fourth-round RTD.^[31][32]