In Round 12 of the 2013 NRL season, Mantellato made his NRL debut for the Knights against the South Sydney Rabbitohs. He scored a try and kicked 3 goals on debut. On 20 August 2013, he re-signed with the Knights on a 1-year contract. On 22 September 2013, he was again named on the wing in the 2013 New South Wales Cup Team of the Year. During 2013, he featured in Italy's successful 2013 Rugby League World Cup qualifying tournament and was selected in the 24-man squad for the World Cup campaign. On 19 October 2013, he kicked the winning field goal in Italy's 15-14 warm up game win over England.