The Kiwi had largely controlled the race at the Brooklyn Street Circuit from pole and had built a solid gap over eventual runner-up Lucas di Grassi, as light rain suddenly intensified and soaked the track.
Cassidy aquaplaned on a puddle on the run to Turn 6 and hit the wall, where he was followed by di Grassi and Stoffel Vandoorne.
The race was red-flagged as a result of the crash as Cassidy's team-mate Robin Frijns had avoided the same incident and, after deliberation from race control, proceedings were brought to an end - the results subsequently taken from the end of lap 29 to hand Cassidy the win.
Although the Envision driver admitted that it was not the way he'd have liked to secure his first win in the championship, he was happy with his pace and felt that he had been able to manage the race despite challenges from the cars behind.
"It feels really, really good - it's not quite the way that we would have liked, but I'm definitely going to take it," Cassidy reflected.
"I've had some weekends in this championship where I'm very competitive, and it's not really worked out. And today's quite the opposite. So I'm pretty thankful.
"It's hard to manage these races, especially when you're leading, it's never easy.
"With the guys behind, let's say there's a lot of tactics and it's not easy to be out front.
"But the pace was strong enough to stay there and we were good on energy, we kind of managed through the run and that made the end of the race quite interesting.
"And obviously Lucas was quite quick, as were Robin and Stoffel. So between the four of us, we had a really nice fight. It got slippery, it got fun. But ultimately, a bit too much."
Cassidy added that he was pleased to get the most out of the Envision package, which has been historically strong in New York City, and felt that he had struggled to capitalise on a strong car earlier in the year.
"If you look at Robin and I, we're both strong here, so I think it trends with car performance. We're lucky that we have a strong car here.
"We knew that coming to the weekend, and so we have to maximise when we've got a strong car.
"I probably didn't do that earlier this season, in the first two or three races. I'm glad that I have done now."
