In the summer of 2017, Cully signed with
MTK Global and turned professional, linking up with trainer Pete Taylor - father of
Katie. He made his debut on 16 September 2017, stopping Hungarian
journeyman Gyula Tallosi inside a minute at the Devenish Complex in
Belfast. A busy start to his pro career saw Cully then fight twice at the
SSE Arena in Belfast, scoring impressive wins over durable Englishmen Josh Thorne and Kane Baker. Following two more undercard wins in Belfast in the first half of 2018, Cully began to step up his level of opposition. First he would score a statement third-round stoppage over game Wearsider Jordan Ellison on 5 October 2018 at the
Titanic Exhibition Centre before knocking out Tanzanian Mohammed Kambuluta (18–5, 7 KO) in the first round at the same venue on 7 December 2018 to improve to 7–0 as a pro. The lilywhite had an uneventful 2019, only fighting twice due to a knuckle gash and missing out on a clash with French champion Renald Garrido as a result. After months of looking for a challenger for the
Irish lightweight title, he finally won the vacant national belt on 1 February 2020 when he faced unbeaten local prospect
Joe Fitzpatrick (10–0, 7 KO) at the
Ulster Hall in Belfast. Cully would stop the
2014 Commonwealth Games silver medalist by first-round TKO for his first title. Cully was expected to face Kieran Gething on 26 August 2020. Gething later withdrew from the bout, and was replaced by Craig Woodruff, who stepped in on 18-days notice. Cully won the fight on points. Six months later, on 13 March 2021, Cully faced Viktor Kotochigov for the vacant
WBO European lightweight title. He made quick work of the Kazakh, as he won the fight by a second-round technical knockout. Cully faced Viorel Simion on 25 June 2021, in his second and final fight of the year. Simion retired from the bout at the end of the third round. ==Professional boxing record==