As a junior he played for the Castleford Panthers, Normanton Knights and he spent time in the Wakefield Trinity Wildcats' Academy System.

Mulhern suffered relegation from the Super League with Hull Kingston Rovers in the 2016 season, due to losing the Million Pound Game by the Salford Red Devils.

[citation needed] 12-months later however, Mulhern was part of the Hull Kingston Rovers' side that won promotion back to the Super League, at the first time of asking following relegation the season prior.

It was revealed on 1 May 2018, that Mulhern had signed a new long-term contract, to remain at Hull Kingston Rovers until at least the end of the 2021 rugby league season.

[6] Mulhern played 27 games for Leigh in the 2023 Super League season as the club finished fifth on the table and qualified for the playoffs.