Education sources told the Herald Mr Leach's decision to leave the school was "eyebrow raising" as it was considered a plum position.
Mr Leach is understood to be joining an educational consultancy, to be run with another ex-principal.
His is the latest in a rash of principal resignations across Auckland - Mangere College, Manurewa High School, Epsom Girls' Grammar, Papakura High School, Papatoetoe High School, Mount Albert Grammar, Ormiston College, Botany Downs and Takapuna Grammar will all have new principals next year.
A letter from the King's College board of governors, published on the school's website today and signed off by chairman Ross Green, said Mr Leach had resigned for "personal reasons" and the board respected his decision.
"The Board of Governors of King's College wishes to advise the King's community that with much regret it has accepted the resignation of the Head, Mr Mike Leach," the letter started.
It added that Mr Leach had informed the board of his likely intention earlier in the term.
"An announcement of his resignation has been withheld until now out of consideration to staff and students during the particularly stressful exam and end of year period," the letter said.
The board then announced the appointment of Mr Lamb as the new headmaster.
"Simon will take up the position at the start of the second term in 2016, prior to which he will work with Mike to transition into the new role."
Mr Leach grew up in Manurewa as the son of a truck driver and spent much of his career teaching at and leading South Auckland high schools.
He was principal at Botany Downs from 2005 and oversaw the growth of the newly established decile-10 school from 600 students to more than 1800.
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.
Takapuna Grammar board of trustees chairman Jon Williams informed the North Shore school's parents and caregivers yesterday of Mr Lamb's resignation.
"I am sorry to have to announce that after 12 years at Takapuna Grammar School, Simon Lamb has decided to move on to a greater challenge," the letter said.
"This is an extremely prestigious role at one of New Zealand's top schools. We should all feel proud that Takapuna Grammar School has prepared Simon for this challenge."
Mr Williams said the school had started the process of seeking a replacement for Mr Lamb.
