The 23-year-old Formula 2 graduate ran 12 of 17 races, which excluded ovals, in a ride share agreement with Takuma Sato.
However five tops 10s, including a best of sixth on the streets of Toronto, established Armstrong as a rookie of the year contender despite the schedule limitations.
It was cemented after a stout recovery to finish eighth in Sunday’s season finale at Laguna Seca, bettering rookie classmate and Juncos Hollinger Racing driver Agustin Canapino by 34 points (214-180).
“I'm happy about it,” said Armstrong, the second New Zealander in three years to win the rookie prize after Scott McLaughlin in 2021.
“The team gave me the best opportunity to do it and I just drove as fast as I could. And that was the result, ultimately.
“At the beginning of the year, my target was to be on the podium and win races, it wasn't necessarily the rookie championship, but that's just a great bonus.”
More will be expected next season after last week's announcement that Armstrong will contest the ovals as part of a full season with CGR.
He will join new signing Linus Lundqvist, defending champion Alex Palou and six-time title-winner Scott Dixon in a four-car CGR lineup, with Marcus Ericsson departing to join the newly renamed Andretti Global operation.
“It was nice to finally talk about what I have in store for the future with Chip Ganassi Racing,” Armstrong said.
“As well, to win the rookie of the year championship, I'm extremely pleased to finally lock that down.
“Obviously, we didn't do five races, so it was going to be a tough ask, for sure. Nevertheless I think we finished every race, we were consistent, we had some good results.
“Sometimes I think we could have done a lot better. Yeah, we were consistent and fast enough to win it without the ovals.”
He added that so far “IndyCar has certainly lived up with its expectations” and says “it's where I want to stay for the next couple of years, for sure”.
Armstrong is set for his first oval test on Wednesday at the high-banked 1.5-mile Texas Motor Speedway.
Having prepared with simulator running and asking veteran Dixon for tips, he said he would “be going there with my eyes wide open and just seeing what I find, making the best of it”.
“I feel like it's going to be a brand-new experience for me,” he said. So, it's exciting. “It's the start of a new journey, I guess.”
