Prince William jumped at the chance to sit on the handbuilt Norton SG7, but joked: 'I'm a father-of-three.
The prince, who took part in a motorcycle rally across Africa with his brother, said: 'I miss the big trips.
The Isle of Man Tourist Trophy is one of the deadliest motorsport events with fatalities every year.
It features riders competing in nine races, from a superbike category to sidecar events, in time trials around the circuit, which is nearly 38 miles long and renowned for its difficulty and danger.
The Prince watched Harrison win the second Supersport race, and presented TT Zero race victor Michael Rutter with his trophy.
'When I said I was going to the Isle of Man for an official visit she said, "Really?" the Duke said at the event.
He spent time with TT staff and volunteers before starting the TT Zero race for electric machines.
Road Racing News magazine described him as a "talented" and "extremely personable" rider, while Gary Dunlop, the son of road racing legend Joey, wrote of understanding "the hurt and pain the family of Adam Lyon will be feeling". This year too, the famous Snaefell Mountain Course, where the participants race, took away the lives of two motorcycle racers. They provide those with disabilities who want to visit the Isle of Man with special accommodations.