Slater commenced playing for the Melbourne Storm in 2003. In Round 1 of the 2003 NRL season, Slater made his first grade NRL debut for the Melbourne Storm against the Cronulla-Sutherland Sharks at Remondis Stadium. Slater started on the wing and scored a try in Melbourne's 36-32 win after overcoming a 22-0 deficit early in the first half. In Slater's next match in Round 2 against the Penrith Panthers, Slater played his first match for the Storm at fullback, scoring 2 tries in the Storm's 42-16 win at Olympic Park. From Rounds 4-8, Slater played in the centre position before moving back to fullback in Round 10 against the Brisbane Broncos in the Storms 36-16 loss at Queensland Sport and Athletics Centre. Slater played the rest of the year at fullback. After a stellar début season saw him crowned Dally M Rookie of the Year (although the awards that year were cancelled in protest). Slater was the Storm's highest try scorer in the 2003 NRL season with 19 tries in their 26 matches.