Masterson was thirteen when Liverpool signed him in 2012 from Dublin side Lucan United. He joined full time in September 2014, starting out with the under 18's and that season he was named Ireland's top under 16 player. On 8th January 2016 he was drafted into the first team squad for the FA Cup third round tie at Exeter, where he was an unused substitute. He was moved up to the Under 23s for the 2017/18 season but failed to develop as expected and made no loan moves. He was released when his deal expired in June 2019 and signed for Queens Park Rangers, but had to be patient and did not make his first appearance until January 2020.

In he first half of 2020/21 Masterson made only three league starts for QPR and came off the bench in just one of the nineteen games in which he was a substitute. On  19th January 2021 he joined Swindon Town on loan for the rest of the season. His spell there was cut short after picking up a hamstring injury in his sixth game which ruled him out for the remainder of the campaign. He failed to make any of QPR's first three matchday league squads at the start of 2021/22 and on 21st August a deal was agreed for him to join League One Cambridge United until January. Masterson started fifteen League One matches before Christmas but with the club not being able to guarantee the same number in the second half of the season, it was decided to allow him to return to QPR rather than extend the spell. On 28th January 2022 a loan deal was agreed with another League One club, Gillingham, where he started eighteen of the nineteen games he was available for. 

Masterson remained with QPR for the first half of 2022/23 but was limited to just one late substitute appearance. He returned to Gillingham on loan for the second half of 2022/23 and was a key figure in helping them pull away from the danger of relegation to the National League. A deal was then agreed to join them on a permanent basis on the expiry of his QPR contract. He had an outstanding season for the Gills in 2023/24, scoring six goals in 41 league appearances and being voted their player of the year. During 2024/25 he had two lengthy injury spells that limited him to 24 league appearances, but when fit he was always in the starting lineup. In June 2025 he signed a one year contract extension and told the club website ""I know the club inside out, I have played my best football here and I am excited to get going again. I have a connection with the fans. We are all hungry now for next season; we know what we want and we'll take it one game at a time and focus on ourselves."