Racing over 180km on a rolling hills course at Te Awamutu on Sunday, Williamson, a member of the Subway-Avanti team, won the junior race by just a second from New Plymouth rider Michael Torker.
All riders from senior down were in the same race, and Williamson rode in the bunch the whole way.
A group of eight broke away reasonably early and, although the bunch started reeling them in, the leading group just held on to claim top honours.
Williamson (19) knew all the riders front of him were not in his grade so he kept himself in the bunch, preparing for the sprint at the end.
With about 500m to go and with the help of a team-mate, Williamson attacked and kept himself in front, negotiating a tricky roundabout to win the grade.
He completed the course in a little over 4hr 20min 13sec, just 50sec down on the senior winner.
He finished sixth overall, with his team-mates Gordon cCauley, Joe Cooper, Jason Allen and Eric Drower, the first four home.
It was the first victory in the under-23 grade in the championships for Williamson, who is now looking forward with his team to the five-day Tour of Wellington, which starts next Wednesday.
Williamson, a physical education student at the University of Otago, said he hoped to take top under-23 honours in the Wellington tour, backing up the under-23 win he took in the Tour of Southland last year.
