The 16-year-old apprentice electrician had been out of action for some time leading up to the nationals after breaking his leg on Waitangi Day.
Henderson came out of his leg cast just six weeks before the Taranaki event and underwent an intensive training regime to be fit for battle.
It was doubly surprising, then, that Henderson also entered the 15-16 years' 125cc class, taking his smaller capacity second bike to finish a solid fourth overall.
He had also been running in second place in that class until the final day.
"I last raced the junior nationals in 2011, finishing ninth overall in the 12-14 years' 125cc class," he said.
"But I'm a lot faster now. I just need to be a bit fitter. I fund myself and have dad as my mechanic, so it's a pretty low-key effort really.
"To be honest, I was pretty surprised at how well I went at the weekend. I thought maybe I'd only get fifth or sixth in both classes."
With his obvious pace, it is probably fair to say Henderson won't be able to "slip under the radar" in the future and that suits him just fine ...
"I want to be out in front. I'm out there to win after all."