He began his career at CITV presenting the children’s television shows Finger Tips (2001–2004) and Tricky TV (2005–2010).

[citation needed] The family spent their holidays at Butlin's Minehead in Somerset, where Mulhern began performing in public.

He became the youngest member of The Magic Circle, which brought about an appearance on Blue Peter in a piece about Harry Houdini.

Turned down at age 18 by Michael Barrymore for My Kind of People, he won another television talent show called The Big Big Talent Show which was hosted by Jonathan Ross, and as a result, in 1997 was invited to take part in the Royal Variety Performance at the Victoria Palace Theatre.

The duo occasionally present the Heart London Breakfast show when regular hosts, Jamie Theakston and Amanda Holden are away.

He covered Graham Norton in the weekend morning 9:30am to 12:30pm slot in August[8] and also presented a week of the Chris Evans Breakfast Show in December.

Since 2013, Mulhern has become associated with the word 'unbelievable' which slowly became his catchphrase, usually used on Britain's Got More Talent as he walks offscreen at the end of a section, with Ant & Dec often mimicking him[11] and is also known for saying "doosh!"

Mulhern regularly presented inside 'The Hub', a small segment on the ITV daytime show This Morning.

[23] In August 2020, Mulhern began presenting a new game show on ITV, Rolling In It, featuring both contestants and celebrities.

In February 2022, Mulhern guest presented Dancing on Ice alongside Willoughby whilst Phillip Schofield self-isolated from COVID-19.

[24] In December 2023, Mulhern was announced as the permanent co-host of Dancing on Ice, as a replacement for Schofield who resigned from ITV earlier in the year.