His last service to the school was as the examinations centre manager for the NCEA external examinations last year.
"Ian leaves a legacy in the school based on strong tradition and values and we will miss him."
Born in New Plymouth, Mr Hamilton's secondary education was at New Plymouth Boys High School before he went on to teachers training college.
He was deputy-head at what was then Wanganui Boys' College from 1976-84 and took over as headmaster in 1984, a position he held until 1998.
A keen sportsman, he served as chairman of New Zealand Schools' Rugby.
After retiring from frontline schooling, he worked for the Education Review Office.
He is survived by his wife Carole, two daughters and a son.
-There will be a service for Mr Hamilton in the City College hall at 2pm on Monday.