The 18-year-old finished second in the fourth and final race of the circuit in Kindberg, Austria, on Monday, but his 330 points was enough to be crowned champion ahead of Fabian Achenrainer (315 points), of Austria, and Italian Philip Haselrieder (255 points).
Leslie, who is from Christchurch but trains in Naseby during the New Zealand winter, won the overall title ahead of 40 athletes from 11 counties.
His time after two runs in the final event was 1min 54.99sec. Florian Markt, of Austria, won in 1min 54.67sec. but Achenrainer, Leslie's biggest threat for the overall title, had to settle for fourth place.
"It feels so good, as the beginning of the season I never thought this would happen,'' Leslie said in a statement.
"I was hoping for at most a top-five finish, but not to finish first. It feels amazing.''
Leslie said his training in the lead-up had gone well, which included him deliberately going slow so his opponents did not know what to expect from him.
"Then for the first race run I had a very good run - all smooth, good lines and fast and I was in first place by three-tenths of a second,'' he said.
"Coming into my second run, I was feeling very confident, but after the first corner I hit a bump that affected the rest of the run. I had the third fastest time for the second run, so dropped to second place, which was still enough for me to secure first overall.''
Leslie's triumph comes just three weeks after he became the first New Zealander to record a podium finish in an international race, when he won the second round at Seiser Alm, Italy.
It surpassed the eighth-placed finish by two-time Olympian Angela Paul at the artificial track junior world championships in the United States in 1995.
New Zealand's other athlete, Leslie's sister, Olivia (16), failed to finish the first of two official training runs and sat out the race with a back injury.
Both athletes will now rejoin the senior World Cup circuit for a race at Deutschnofen, Italy, before competing in the junior world championships in Italy next month.
It is the last year Leslie is eligible to compete in the junior world championships.
