Mellor is the son of former Manchester City, Norwich City, Sheffield Wednesday and Brighton and Hove Albion player Ian Mellor. A strong, physical, centre forward, Mellor was top scorer in Liverpool's run to the FA Youth Cup semi-final during the 2000–01 season scoring eight goals in four games. He was also the top scorer for the under-19 team and Joe Corrigan's reserves during the 2001–02 season and in total scored 46 goals (56 including friendlies). Mellor progressed through the ranks at Liverpool and in the 2002–03 season he scored 20 goals for the reserve team. He made his first team debut in the same season and played a total of six games and scored in a League Cup semi-final match against Sheffield United. Although the Reds lost the match 2–1, Mellor showed promise and was rewarded in March 2003 with a new three-year contract running to 2006. Despite his goal he was left out of the squad for the victorious 2003 Football League Cup Final.