Mulhern is the third of four children of Irish parents, Maureen and Christopher Mulhern, who were both market traders. He has two older brothers and a younger sister. Mulhern attended St Bonaventure's Catholic School in Forest Gate, Newham. Having been interested in magic from an early age, Mulhern gained his interest in comedy and magic from his father, who would teach him a trick before bedtime when he was 13 years old. The family would spend their summer holidays at Butlin's Minehead in Somerset, where Mulhern first began performing in public. He became the youngest member of The Magic Circle, which brought about an appearance on Blue Peter in a piece for Harry Houdini's 100th birthday. Mulhern was later suspended from The Magic Circle for revealing a magic trick on Finger Tips, a kid's TV show.''I was doing a kids TV show called 'Finger Tips' with Fearne Cotton. I think if you don't show kids how to do a trick then you'll never get a new breed of magicians, so I suggested we teach a new trick on the show" He said about the ordeal in 2015. 

After finishing the school, he had a magic show called Stephen’s Mega Magic Show and he spent two summers while working at the Butlins Red Coat show in Minehead. He got the winter job in Harrods and Hamleys, where he met Princess Diana there. When he was 18 Stephen applied to appear on Michael Barrymore’s My Kind of People but was turned down.  He went on to win a TV talent show hosted by Jonathan Ross and was subsequently invited to take part in the Royal Variety Performance in front of The Queen, Elizabeth II. He was given a job on the Disney Chanel and progressed onto CITV alongside Holly Willoughby on Saturday Showdown. He has since appeared on numerous magic-based TV shows including The Quick Trick Show, Tricky TV, Freaky, and more recently Magic Numbers. 

Stephen’s television career really took off as a CITV presenter, presenting shows such as Holly & Stephen’s Saturday Showdown, alongside Holly Willoughby (nominated for two Children's BAFTAs), Tricky, Fingertips, Globo Loco, the BAFTA award winning Quick Trick Show and Ministry of Mayhem.  

In 2013, ITV brought back a revamped version of the popular quiz show ‘Catchphrase’ of which Stephen is the presenter. He also fronts ITV’s Big Star’s Little Star, a primetime show in which celebrities and their children compete to win money for charity. Stephen also presents a section of the incredibly Popular Ant and Dec’s Saturday Night Takeaway, where Ant and Dec go head to head in a series of challenges.