The three-cap front-rower has taken the step to help remove stigmas around sexuality for rugby players of younger generations.
The 43-year-old won his three All Blacks caps in 2005, spending four years with the Crusaders, then three years in France with Biarritz.
"If I can be the first All Black that comes out as gay and take away the pressure and the stigma surrounding that whole issue then it can actually help other people,” said Johnstone.
”We’ll know that there is one amongst the All Blacks."
The All Blacks issued a message of support for Johnstone, praising him for "having the courage to share his story and helping create a more inclusive game".
While Johnstone did confide in those closest to him on his sexuality during his playing days, the accomplished prop has now admitted retaining his privacy more widely could have held him back.
"I’d push that side of me down deeper and deeper,” said Johnstone. “It would come to the surface when I may have had a bad game and I would look at that side of me and blame that side for it.
“It slowly starts to affect you and it’s hard living a double life or living a lie.”