Apart from claiming the title, they moved four points clear of Australia on the series points table on 58 points to 54.
The win capped a memorable victory for the side, who conceded only two tries, both to Japan, over the weekend.
Captain Sarah Hirini said she was stoked with the defence throughout the weekend.
"We've got some amazing people, and people who can outrun other people," she said.
In the final, the USA struggled with the inside passes, with New Zealand wing Michaela Blyde scoring from the first switch of direction resulting from a Jorja Miller midfield steal.
New Zealand's defence was stunning throughout, and Jazmin Felix-Hotham was provided with her scoring chance after Tyla Nathan-Wong held an American up to win a scrum turnover.
Felix-Hotham linked with Blyde on the inside again to start the second half. Miller almost scored an unlikely length of the field try only to knock on at the last gasp. But, while USA had the scrum feed, they were pressured on their line, and when the Black Ferns received a five-metre scrum, Blyde completed a hat-trick of tries and scored her 10th of the weekend.
While the USA scored, it was too little to affect the game's flow, and New Zealand polished the win with a try to replacement Portia Woodman-Wickliffe.
Ireland rarely had an attacking chance in their semifinal, twice resorting to kicking the ball ahead when unable to penetrate the Black Ferns' defence. However, both kicks failed to yield results as New Zealand easily accounted for them.
Jorja Miller's footwork undid the Irish as she crossed for both the Black Ferns' first-half tries. But in the second half, the home side unleashed their speed.
Stacey Waaka-Fluhler caught the restart to the second half, fed the ball to her left and was in perfect position on the right flank when it came back to her to extend the lead. Michaela Blyde followed that with another try when New Zealand called for a scrum and had space on the left.
Portia Woodman-Wickliffe was introduced and demonstrated the power of her fend on the left flank when challenged to push on to score, with Blyde following on full-time when shimmying out of a tackle attempt as they claimed a 32-0 win.
New Zealand's Black Ferns Sevens proved too strong for Japan in their quarterfinal, beating them 43-12.
Japan responded to the Blacks Ferns' first try, scored by Risi Pouri-Lane in the second minute. But when Miller scored in the seventh minute, and Shiray Kaka scored the first of her three tries in the eighth, it was all over for Japan.
New Zealand led 19-5 at the turn, and Kaka crossed immediately while Pouri-Lane and Kaka scored again, with Woodman-Wickliffe rounding out the game with a try.
In the men's side of the draw, the All Blacks Sevens fell agonisingly short to Argentina 14-12 in a dramatic final.