Racing at the Mid Ohio Sports Car Course in Lexington, Ohio, Leitch won the rain-affected opening race and backed it up with a third and a second in the following races to claim the outright round victory.
Leitch told the championship's website he was able to call on his seasons of racing karts and single seaters in New Zealand's wet conditions to master the tricky first race.
''Having experienced racing in the rain really paid off for us today. It rains quite often in New Zealand so I race in conditions like this a lot. My team worked extremely hard to get me to the podium. My first win in America has been poetic,'' he said.
Leitch (22), who works at Highlands Motorsport Park as a mechanic and hot lap driver, is contesting the United States-based championship with the Kiwi Motorsport team. He believed a savvy race strategy helped him rocket from 19th in a first-lap melee to first at the chequered flag, and credited the victory to using slick tyres on a wet but drying track.
''We made a brave move to go out on slicks and as the race progressed I pushed through the field, setting fastest lap of the race, and came away with the win,'' he said.
Starting eighth on the grid in that first 30-minute race, he surged through to the early lead but was then caught up in a multiple-car tangle and regained the track in 19th overall.
''I thought it was all over at that point,'' Leitch said.
Battling through the field, he overtook car after car on a drying track and climbed all the way up to fifth overall with two laps to go. Then heavy rain started to fall, the leader slid off the track, and Leitch was able to exercise his skill set when it got slippery.
''I kept it together and gently brought the car home for our first win. I'm totally rapt,'' he said.
The Kiwi driver then capped the victory with third in the second race despite an early spin, then finished second in the final race, conclusively announcing his presence as a force in the championship.
Leitch started the weekend in 16th place in the standings but the round win propels him into fifth, with three rounds remaining.
His medium-term goal is still to build contacts in the United States with the aim of testing and securing a drive in a major United States-based single-seater category.
