McNab previously won world titles as a 6 and 7-year-old, in South Africa and Copenhagen, but missed out last year to the American.
Olympic Silver medallist, Kawerau's Sarah Walker tweeted after the race congratulating the young rider.
"Lachie with an incredible win in the 9yr boys!! ... Kiwi domination! Awesome results for session 1 today."
Lachlan was one of six riders from the Rotorua BMX Club who made their age-group final in the U10 races yesterday.
Megan Williams finished second and Jenna Hastings finished third in 9 girls, Wyatt Stevens-McNab finished third and Ryan Hastings finished seventh in 7 boys, and Mila Alaeinia finished eighth in 5-7 girls.
Rotorua BMX Club vice-president Craig Elliott said there was a great atmosphere at the world champs, with massive support for all 47 riders from the Rotorua BMX Club.
"Everyone has been on their feet cheering them on," he said. "Every rider from the club has just been yelled at it in support, it is awesome."
He said all the riders who had competed so far had done themselves proud. "Every Rotorua rider has put something into the New Zealand fern."
Elliott said a lot of the group from Rotorua had lost their voices from all the cheering.
During the first day of racing on Wednesday, Rotorua BMX Club riders Dave Mohi and Zander Steel also managed placings in their category final.
Mohi finished fifth in the 30-plus men's event and Steel finished sixth in the 17-24 men's event.
Then yesterday Mohi became the second Rotorua BMX Club rider to claim an age-group world title at this year's championships.
Mohi won the 40-44 men's title during the second session yesterday at the world champs in Auckland.
Mohi said he was absolutely rapt with the win.
"I have trained really hard. I am generally not very good at gates so I have been smashing them in training," he said.
"It was mean to race at home because the crowd was just nuts."
This is Mohi's second age-group world title.
Racing continues at the Vector Arena in Auckland today, with the elite competition held tomorrow and Sunday.
New Zealand now has 12 world titles in two days, ahead of USA and Australia who have six each.
The world champs continue into the weekend with the feature races - the men's and women's elite races. Kawerau's Sarah Walker is set to race in the women's elite competition.
The track will be converted today from its current set-up to an 8m start gate and bigger jumps for the elite competition.
BMX World Championships schedule
Friday
4pm - Junior men practice
5.30pm - Junior and elite women practice
7pm - Elite men practice
Saturday
9am - Junior men practice
10am - Junior and elite women practice
11am - Elite men practice
1pm - Time trials begin
5pm - Time trial super finals (top 16 elite men and women, top eight junior men and women)
6.15pm - Podium presentations
Sunday
8am - Warm-ups
10am - Racing begins, three motos for each group (top 16 elite men, top eight elite women exempt).
3pm - Elimination rounds begin
5pm - Podium presentations