His contract was already extended by two years after he served two three-year terms, which is standard for the heads of government departments.
Sanson plans to work in environmental and social governance, and will be volunteering with the Predator Free 2050 movement and the Backcountry Trust.
He will step down in September.
In a statement, Sanson said he had enjoyed working in the department, having been involved in conservation since 1971 when he started out as a track cutter.
"I was particularly proud of the work to establish Rakiura National Park and the New Zealand Sub-Antarctic Islands World Heritage Area as Southland Conservator from 1996 to 2002.
"The work I'm most proud of is the work that has made a difference for people and for conservation. Especially how much we have shifted our critical relationships with whānau, hapū and iwi around Aotearoa modelled on Doc's Te Kaupapa."
The injection of $500 million for the Mahi mō te Taiao - Jobs for Nature scheme was also a huge contribution to conservation efforts, he said.
He expressed his hopes to see Wellington become the world's first predator-free capital city.
"I would love to hear kiwi in our capital city."
He thanked his family for their support as well.
