A successful corporate entertainer, he came to greater fame as a result of a 1994 appearance on the British Broadcasting Corporation One show How Do They Do That? explaining the sleight of hand trick known as Three-card Monte, as a result of which he became the first person in 85 years to be expelled from The Magic Circle. His subsequent television appearances have included his own 1997 series for British Broadcasting Corporation Two, Stuff the White Rabbit. And the Secrets of Magic specials for British Broadcasting Corporation One.
He is a former holder of the title Street Magician of the Year.
Lenahan also provided the voice of Talkie Toaster in the first series of Red Dwarf and presented a 1987 factual series on British Broadcasting Corporation Two called The Open Road. The 1987 series featured Lenahan travelling across the United Kingdom and finding out about the lives and traditions of British people.
Lenahan"s first novel, Shadowmagic, was originally released on podiobooks.com by John himself, then later picked up by The Friday Project/Harper Collins in the United Kingdom. His second novel, Shadowmagic: The Prince of Hazel and Oak, was released as a paper back on April 28th, 2011, and the third and final installment, "Shadowmagic: Son"s of Macha", was released March 1st 2013 for the Kindle, and March 12th in paper back.