Nonu, who was head coach at Redruth last season, will take up the role in April, replacing James Reyburn, who has moved to New Zealand.
Nonu will move to Mount Kelly from Camborne Science International Academy, where he has been teaching physical education.
Mount Kelly headmaster Guy Ayling said: “We are delighted that Palepoi will be joining the Mount Kelly community. Not only does he come with rugby credentials of the highest order, his emphasis on a values-based coaching style is a perfect fit for us and I know that our young people will benefit enormously from his influence both on and off the pitch.”
“It is hugely important that the values and individuality of our pupils are enhanced. I look forward to helping and being a part of what I consider to be something very special indeed.”
As well as turning out for Albion, New Zealand-born Nonu, who has a first-class honours degree in sports performance and coaching from the University of Plymouth, has also played for Launceston, Rotherham and Redruth in England having learned his trade in New Zealand at Wellington.