Manager Gérard Houllier sent Mellor on a year-long loan to West Ham United in August 2003. He scored two goals in the 3–0 win over Crystal Palace at the end of September, but an injury hit spell at Upton Park, in addition to playing under three different managers and being forced to play out of position, proved unsettling. Mellor was soon back at Anfield and hit 10 goals in just four games for the reserves. 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. In the next round he scored two goals to knock out holders Middlesbrough, and in late November he started a UEFA Champions League game at Monaco.