Driving the No. 27 NAPA AUTO PARTS Honda, Rossi cruised across the finish line 20.2359 seconds ahead of Team Penske's Josef Newgarden - the largest margin of victory at Long Beach since Unser Jr. won by more than 23 seconds in 1995.
Four of Rossi's six NTT IndyCar Series career victories have now come when the 27-year-old Californian started from the pole position. Sunday's win also moved him into second place in the 2019 championship standings after four of 17 races.
"You never think (you'll dominate the race)," said Rossi. "We knew it was going to be a really hard-fought battle. The NAPA AUTO PARTS crew gave me such a great race car. This is the 200th win for Andretti Autosport, so what better way to do it than in that type of fashion here at Long Beach (and) here in California."
Rossi also delivered Andretti Autosport its 200th race victory and sixty-three of those wins have come in IndyCar Series competition. Team owner Michael Andretti had high words of praise for Rossi after the race.
"I think he's a great driver," Andretti said. "I think we saw in him from the moment he sat in one of our cars, we knew we had someone special here. He's been great to have as a part of the team. He drove flawlessly all weekend. That lap he did (in) qualifying, I think, was the difference as well. To be able to start on pole here is a big advantage."
"It's an amazing day," said Rossi. "I have a great car and a great crew behind me. I couldn't do it without them, so hats off to them. This is a special one. I just found out my grandfather died yesterday, so I wanted to dedicate this (win) to him and obviously Michael (Andretti) for 200th win."