Before that, he taught at nearby schools including James Cook High School, Edgewater College, Papakura High School and Tuakau College, before becoming principal of Otara's Tangaroa College.
Tangaroa, a decile-one school, draws its students from one of the poorest communities in New Zealand - a stark contrast to King's, where boarding fees are about $36,000 a year.
Talking to the Herald in his wood-panelled office, Mr Leach, a former Counties Manukau representative rugby flanker, said all schools were grappling with the same developments.
"I always find that no matter where you are, kids genuinely want to do well.
"Education is becoming more personalised ... Finding the right pathway for each kid is almost a necessity now - you no longer teach large groups of kids the same thing.
"I think that, in terms of education, it is a watershed time, with the changes in pedagogy and in using technology."
Mr Leach's wife, Jenny, is principal at Otara's Ferguson Intermediate, and he has two grown daughters.
When previous headmaster Bradley Fenner, an Australian who was at the helm for five years, started at King's, he found an old cane in his office cupboard.
What can today's students expect when they are summoned to see the new headmaster? The early signs are promising.
"There are very few of us who have grown up without making lots of mistakes, so adolescents are always going to do that," Mr Leach said.
"It's about them saying, 'Sir, how do I fix this up?'
"And as soon as a kid is doing that, you know that they are great. There doesn't need to be anything else."
Michael Leach
*New headmaster at King's College, a private school founded in 1896.
*Becomes the first King's headmaster who has been chosen directly from the state school sector.
*Mr Leach, 57, grew up in Manurewa as the son of a truck driver and school office worker.
*Ex-principal of Tangaroa and Botany Downs Secondary colleges.
