Mulhern featured heavily for the Lions in pre-season ahead of the 2017/18 season on trial and was rewarded a contract.
He joined Huddersfield Town’s under-23 development squad in February 2016 from Leeds United.
Capped by the Republic of Ireland at under-18 and under-19 level, Mulhern, who hails from South Elmsall, was in the Huddersfield academy for nearly four years joining the Terriers at under-9 level in August 2005 before departing for Leeds in May 2009.
He subsequently turned professional with the Elland Road outfit in the summer of 2015 following a two-year scholarship; a deal he extended a year later.
He had a loan spell with then-National League side Southport during September 2016 and then spent the last month with the Reds` league rivals Harrogate Town, featuring twice.
His brother Robbie is a professional Rugby League player for Hull KR.