In 2012, Mantellato joined the Newcastle Knights' New South Wales Cup team, finishing the season as the competition's highest point-scorer, with 306 points. On 12 September 2012, he was named on the wing in the 2012 New South Wales Cup Team of the Year. He then signed a 1-year contract with the Knights.
On 11 August 2014, Mantellato signed a 3-year contract with Super League club, Hull Kingston Rovers, starting in 2015. He made his Super League debut in Round 1 against the Leeds Rhinos. He kicked off his Super League points tally by kicking 5 goals in their 30-40 first round defeat. He scored his first two Super League tries against the Warrington Wolves in Round 3.