Mellor signed as a professional for Wigan from Warrington-based amateur side Latchford. In 2009, Mellor was awarded Man-of-the-Match in the Reserve Grand Final following a fantastic performance. An injury-plagued 2010 meant limited appearances in Wigan's junior teams, however, he did play in the Under-20 Cup Final win over St Helens.
On his return to Wigan, Mellor was sent on loan to Widnes along with Dom Crosby and Logan Tomkins. Mellor featured at stand-off for the Chemics, scoring tries in the home fixtures against Barrow and Toulouse Olympique, and a brace away at Barrow.
Mellor was once again loaned to Widnes in 2012, and joined the club on a permanent contract after being released by Wigan at the end of 2012.
After the departure of captain Kevin Brown to the Warrington Wolves at the end of 2016, Mellor was handed co-captaincy alongside Chris Houston, and has been given the number 6 jersey.
Leigh
On 10 November 2020, it was announced that Mellor would join Leigh for the 2021 Super League season.[4]
On 28 May 2022, Mellor played for Leigh in their 2022 RFL 1895 Cup final victory over Featherstone.[5]
On 3 October 2022, Mellor played for Leigh in their Million Pound Game victory over Batley which saw the club promoted back to the Super League.[6]
On 12 August 2023, Mellor played for Leigh in their 2023 Challenge Cup final victory over Hull Kingston Rovers. It was Leigh's first major trophy for 52 years.[7]
Mellor played 23 games for Leigh in the 2023 Super League season as the club finished fifth on the table and qualified for the playoffs. He played in their elimination playoff loss against Hull Kington Rovers.[8]