The regional field manager from Rotherham, South Yorkshire, said: "It was the TT senior race, I think they were on lap two.
"The bikes were coming straight at him but it was just as if it didn't faze him. He's a danger mouse.
"It would just stop, sit down and chill, then goes for it again. It was very strange and very tense to watch.
"When the wind knocked it over I thought, that's it, it's had it. I didn't think it would make it."
Alderson said the tension of the race was diverted to watching the fate of the little daredevil - who decided to cross the circuit and one of its busiest points - off a bend at Ballig Farm.
The petrolhead Peter said: "The bikes were travelling about 120 miles per hour, and they're just coming out of a bend so they're building up speed again. They're full-blown race bikes. The circuit's the Holy Grail for bikers.
"So it's a bit of a miracle mouse. People see mice around the circuit but I personally haven't seen one do that before."
Alderson, who has had a life-long interest in motorbikes and was attending the Isle of Man event for the third time, said he and his companions screamed at the mouse to move.
And they were all delighted and amazed to see him miraculously reach the other side of the road in one piece.
He added: "It's not one of the main viewing spaces for the race, but there were a few of us there watching the mouse and shouting at it to move, not that it could understand us.
"I don't know if any drivers could see it, they probably thought it was a leaf or something. I think five or six bikes went past before it got to the other side.
"It kind of came over and went round a little wall. When it was off the circuit I went and picked it up and took it further down into the woods.
"Everyone was joking later that we could have just fed the farm cat."
