Graham Rahal was third while championship contenders Simon Pagenaud and Castroneves finished fourth and seventh respectively.
"I got a great crew," Newgarden said. "When they give me a good car like this, I don't want to say it is easy but it is easier at least.
"It was a really fast car - reds or blacks it didn't matter."
It is the first time an America driver has led the Indycar series since Ryan Hunter-Reay led briefly in the middle of the 2014 season.
"That's great," the Tennessee native said. "We just have to keep it up. No mistakes - if we don't have any mistakes we have enough to win this championship."
"Really bizarre - right from the get-go," the Kiwi said. "The car just had no rear end all day. I was two or three seconds a lap off.
"As soon as I turned the wheel it just wanted to spin out. I think something mechanically is broken on the rear of the car.
"It was just one of those days where everything that could go wrong did go wrong.
"It is frustrating but that is how it goes sometimes.
"We are fortunate to be where we are - it is still very tight pints race.
"Congrats to Josef.
"We had our hands full today but I think we can still turn it around."
There are four races left in the championship.
