In December 2011, a knee injury sustained following a tackle by Milton Keynes striker Jabo Ibehre resulted in Mellor being forced into announcing his retirement on 9 May 2012.

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.

Before the start of the 2004–05 season, Mellor suffered a knee injury but regained fitness and a return to the first team in the 3–0 League Cup victory at Millwall.

[10] Things got better still for Mellor ten days later when Liverpool were being held 1–1 by Olympiacos in a game they had to win by two clear goals to progress to the Champions League knock out stages.

After his retirement from professional football, Mellor returned to Liverpool to do media and commentary work with their in-house television company LFC TV.

In August 2006, Mellor signed a three-year contract with Football League Championship club Preston North End for an undisclosed fee, thought to be in the region of £1.5m.

Despite the promising pre-season and Mellor being a favourite amongst sections of the Deepdale crowd, indifferent form led to him losing his place to new signing Jon Parkin.

Mellor regained his place in the team in time to score in Preston's 3–1 victory away at local rivals Blackpool, but another injury saw him miss a number of games.

[19] On 2 July 2010, Mellor rejoined former manager Alan Irvine at Football League One side Sheffield Wednesday on a season long loan.

[20] He scored his first professional career hat-trick for Sheffield Wednesday on 10 November 2010 in a 4–1 win at Hillsborough over Hartlepool United in the Football League Trophy.

A few days later, he started another home match, this time against Tranmere Rovers setting up two first half goals, before winning a penalty, of which Owls captain Tommy Miller duly converted.