Speaking ahead of the release this week of the first volume of his two-part autobiography, My Previous Life in Comedy, he explained how he got his lucky break on TV after starting out as a comedian.
In his jovial style, he said: "I turned up for the audition in a tailor-made suit and Russell & Bromley shoes, which were £70 back then.
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"When I arrived, I thought the audition would be for the show Crackerjack. I got the job.
They said to me, 'You are going to be wonderful in Play School'.
"It was for under fives at 11am on BBC2 but nobody had BBC2 back then. I guess it is always the same in showbiz, you always get the job you don't want." At first, he struggled, until he began using his stand-up talent to impressive effect. He went on to front some of the most loved children's shows of the 70s and 80s.
In their heyday, Play Away, Think of a Number, Think Again and Johnny Ball Reveals All were watched by audiences of more than five million.
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And along the way he showed children how to fall in love with science and maths, with adults also tuning in to expand their minds.
He said: "It was my comedy training that helped me write factual info scripts that were never boring but always had a joke or a fun idea at every turn.
"I wrote most of the sketches for Play Away. What you have to do is take the audience away and do them as adult sketches, just cleaner, that's all it was." Johnny said he first crafted his comic timing as a Butlin's Redcoat in Pwllheli, north-west Wales, but also credits his dad, Danny, an iron founder, for passing on his comic talent.
He added: "It was my dad who led me to appreciate comedy. I have always said that if he had the chance, he would have been a wonderful comic.
"He couldn't utter a straight line. He was in the Depression and the war and the cards never fell in his favour his entire life. However, that didn't stop him saying something funny all the time." Speaking at his Buckinghamshire home, Johnny, whose second book will be out next year, said his dad also took him to see stars including George Formby at the Blackpool Opera House.
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After a spell working in accounts for the De Havilland aircraft company and three years in the RAF, Johnny started landing regular comedy slots.
He supported Dusty Springfield, and The Rolling Stones, an experience he would rather forget.
"It was terrible," he said. "The fans screamed and screamed and nobody heard me for a fortnight." He turned down chances to do BBC talent show Opportunity Knocks, instead going on to demystify maths.
He said: "We had such ambition on those shows. We never thought we were broadcasting to a children's audience, just the people out there.
"The BBC found out our audience was 70 per cent adults. So what they did was they started to make shows like Grange Hill and Byker Grove, which taught kids how to be naughty, which they already knew how to be.
"All our shows, Roy Castle's and Tony Hart's, which tried to help children understand the world and science and technology, were thrown out of the window and it was so sad." But Johnny, who is back from a skiing holiday with his second wife Diane, will be happily back on TV this week promoting his book, with slots on Loose Women, The One Show and Saturday Live.
He said: "If I go into a studio at 86, I a...