Johnstone, who played three tests in 2005, had already told his family and close friends before publicly coming out on New Zealand current affairs programme Seven Sharp.
"If I can be the first All Black that comes out as gay and take away the pressure and stigma surrounding the issue it can actually help other people," Johnstone said.
"If I open up that door and magically make that closet disappear, then we're going to help a lot of people... "
"I pushed that side of me down deeper and deeper. I went to some interesting places."
Johnstone made his debut for New Zealand against Fiji and played two tests against the British and Irish Lions. He also made 62 appearances for Canterbury and 38 for the Crusaders.
New Zealand Rugby chief executive Mark Robinson, who is also a former teammate, said in a statement Johnstone's "strength and visibility will pave the way for others in our game".
"We know that there are people who have not always been comfortable to be who they are in rugby. We want to be clear, no matter who you love, rugby has your back," Robinson added.
New Zealand Sports Minister and former deputy Prime Minister Grant Robertson, who is gay, thanked Johnstone for "blazing this trail" and said his announcement was "a big moment".
"Another barrier has been broken. I hope it inspires future generations to be open, happy and comfortable," Robertson wrote on Instagram.
"There is still a long way to go, but feels a very significant step."
Savea praises Johnstone's 'courageous' decision
Ardie Savea (29) said Johnstone was "pretty courageous" for deciding to publicly come out as the first gay All Black.
"It takes a lot of courage, especially in the rugby circles and how it's been in the past; the old traditional hard-man. But seeing someone come out, it's pretty courageous," All Blacks loose forward Savea said on Wednesday.
"I know the doubts that would've been in his head, but he probably doesn't realise how many people he's helped internally with what he's done, so big ups to him."
Savea said the All Blacks were "pretty open".
"We're all about togetherness, regardless of if you're Samoan or Pacific, if you're gay or if you're straight, you're accepted and loved."
