Following Bradford's liquidation before the 2017 season, he signed a three-year deal with Super League side Huddersfield.

On 17 October 2020, he played in the 2020 Challenge Cup Final victory for Leeds over Salford at Wembley Stadium.

[5] On 21 June 2022, the Castleford Tigers announced that Mellor would join the club with immediate effect, having signed a two-and-a-half-year deal.

[6] He made his Tigers debut later that week, in a golden-point win against the Catalans Dragons at Wheldon Road.

[13] Mellor made 25 appearances throughout the season as the club narrowly avoided relegation by finishing above Wakefield.

"[14] Mellor suffered a knee injury in a friendly against Keighley ahead of the 2024 season, forcing him to miss the opening two rounds.

[15] He displayed impressive form as the season progressed with Castleford director of rugby Danny Wilson praising his "tough line-running",[16] and subsequently signed a two-year contract extension in May.