In 1995, Mellor joined the cast of the soap opera, Hollyoaks in which he played the role of James 'Jambo' Bolton. He left the show in 1998. Soon after, he released a single called When I Need You, which reached number 5 in the British charts, as well as a follow up single called No Matter What I Do, which reached number 23.

He also made an appearance in the music video for Boyzone's Comic Relief single "When The Going Gets Tough".

Shortly after, he returned to acting, playing Greg, in Is Harry on the Boat?. He also played Jack Vincent in Casualty between 2001 and 2003 and has starred as Gaz Wilkinson in the comedy Two Pints of Lager and a Packet of Crisps since 2001. He also made a brief cameo reappearance in a Christmas edition of Hollyoaks, in which he played the ghost of Jambo (although the character isn't dead). Will also stars as "Barmpot" in postal worker drama Sorted and has also played DC Jed Griffiths in the second series of Murder Investigation Team. In 2004 he provided the voice-over for Georgie (the dog) in the CITV series Barking.

Mellor won the first series of the BBC singing contest, Comic Relief does Fame Academy in 2005. Mellor made his EastEnders debut on 20 May 2007 playing Warren.

He is probably best well know for his part of Gaz in Two Pints Of Lager.