He was expected to put his hand up again after English resigned earlier this year, but decided against it and was already in talks with Acurity.
Failing to secure the leadership had nothing to do with his decision, he said.
"I had a good go at the end of 2016 so I never died wondering. My priorities have shifted.
"When we lost the last election, it was my view that there would probably be a leadership change at some stage, and I was quite keen in my own mind that I didn't want to contest the position of Leader of the Opposition."
Coleman, 51, said leaving was a "tough decision", but it was about going out on top and creating a successful career after politics.
"I would be staying to possibly get my old job or an equivalent back. It's time for me to follow a new future."
He told National Party leader Simon Bridges about his resignation yesterday.
Bridges thanked Coleman for his many years of dedication and service to the National Party and New Zealand.
"He has been a diligent and committed colleague who has handled a range of challenging portfolios with a focus on getting results. My colleagues and I wish Jonathan and his family the very best for this new phase of their lives."
Coleman has held a number of ministerial portfolios, including Defence, Immigration and State Services - but his most controversial was Health.
He said you can never please everyone in the health sector, and listed a series of achievements including decreasing cancer waiting times, decreasing ED waiting times, which saved 700 lives a year, and increasing elective surgery numbers.
"But there will always be critics."
He was in charge of the portfolio during a growing chorus of critics about mental health services, demand for which had almost doubled in the last eight years.
"Mental health is an area that in every western jurisdiction in the last two years has become a sudden focus for Governments."
He denied that his resignation was a disservice to the people of Northcote, saying local constituents he had spoken to had been supportive of the decision.
"After 13 years of public service, people are very supportive."
Coleman entered Parliament in 2005, but a year later, as National's associate health spokesman, drew criticism for accepting an invite to British American Tobacco's corporate box and smoking a cigar at a U2 concert.
Coleman apologised and accepted it was wrong for an associate health spokesman to be seen smoking.
"I didn't even get to finish the thing anyway. It's a very occasional thing, and I can tell you I've lost the taste for cigars over this whole incident, that's for sure," he said at the time.
