Ngatai recovered to regain his place at the Chiefs and to represent the Maori All Blacks at second five-eighth against the British and Irish Lions in 2017.
He left Hamilton for Lyon at the end of the 2018 Super Rugby season and chalked up 88 appearances.
Lyon, captained by former Crusaders backrower Jordan Taufua, finished ninth in the French Top 14 this season, but won the Challenge Cup — European rugby's second tier — beating French rivals Toulon 32-10 in the final in Marseille last month.
Now Ngatai has joined Leinster.
Leinster director of rugby Leo Cullen, had had Ngatai in his sights since 2014 when he saw him at work with the Chiefs.
He reckoned Ngatai would have won more All Black caps but for his injury issues in New Zealand.
Gisborne-raised Ngatai (Ngati Porou/Te Whanau-a-Apanui/ Te Whakatohea) made his professional debut in 2009 with Wellington after representing Poverty Bay as an 18-year-old — scoring in the Bay's 26-5 Heartland Championship Lochore Cup win over Horowhenua Kapiti in 2009.
He also played provincial rugby for Taranaki, captaining the side.
Ngatai initially played Super Rugby for the Hurricanes but made his mark at the Chiefs, where he played 56 games from 2013.
He was at centre for the 2013 Super Rugby final victory over the Brumbies and later captained the Chiefs.
Ngatai earned 12 caps for the Maori All Blacks and captained the side.
He won his sole cap for the All Blacks off the bench against Samoa in Apia in 2015.
Ngatai seemed set to challenge for a place in the All Blacks midfield after the departure of Conrad Smith and Ma'a Nonu following the 2015 Rugby World Cup, but he was out for almost a year after that concussion from the head knock against the Hurricanes.
Then came recovery and the start of a new phase of his career in Europe.
Ngatai faces stiff competition at Leinster, who boast Ireland test midfielders Garry Ringrose and Robbie Henshaw, but Cullen is delighted to finally get his man.
“I first came across Charlie back in 2014 when I visited the Chiefs late-on in my playing days and as I was preparing to make the transition into coaching,'' Cullen said on Leinster's website.
“Even then, you could see the influence he (Ngatai) had on that Chiefs squad and that influence I think has only increased over the years with Lyon most recently.
“He's a New Zealand international and I believe would have been capped far more but for his injury issues, but he has been back playing a few years and you could see even in the Challenge Cup final in Marseille just how influential a player he is.”
Ngatai will arrive in Dublin ahead of pre-season training in the northern summer.
He will join New Zealand-born Ireland internationals Gibson-Park and James Lowe and former Crusaders prop Michael Alaalatoa at Leinster.
Gibson-Park and Ngatai were teammates in Taranaki's 2014 NPC grand final-winning team and Lowe played against them for the Tasman Mako.
Leinster are one of Europe's top clubs, having won four European Champions Cup titles and eight PRO12 or PRO14 club championships.
