Mo'unga has moved to Toshiba Brave Lupus on a three-year deal, meaning he cannot play for the All Blacks during that time unless eligibility rules change to allow overseas-based players.
The 29-year-old, who played a pivotal role in New Zealand's run to last month's World Cup final, said he was "fully involved" in his move to Toshiba.
"I'm focused on the present, I just want to give all I can to Toshiba," the fly half told reporters.
"I really can see myself playing here as long as I can until I retire."
Mo'unga is one of several All Blacks moving to Japan, including captain Sam Cane and newly crowned World Rugby player of the year Ardie Savea.
Flanker Shannon Frizell has also joined Mo'unga at Tokyo-based Toshiba.
Cane and Savea have signed short-term deals that mean they will not miss any games for New Zealand.
In Mo'unga's case, he said that "at this very point, the All Blacks is over there".
"I don't like using the term 'sabbatical'," he said.
"This isn't a rest for me, it's not a one-year thing. I'm fully involved and fully committed."
Mo'unga was outstanding as New Zealand shrugged off a difficult build-up to reach the World Cup final, where they lost 12-11 to South Africa.
He said he believes he can play "the best footy of my career" at Toshiba, who finished fifth in last season's Japan Rugby League One table.
The new campaign kicks off on December 9.
Mo'unga said he was looking forward to a change of scenery after playing for Canterbury and the Crusaders in his native Christchurch.
"Every year for a long number of years, it's been the same, same, same," he said.
"This is an experience for myself to test my abilities in a different environment."
