The 23-year-old hadn't finished higher than 35th at a major, but the final two rounds at TPC Harding Park in San Francisco propelled him to victory. But just two strokes behind is Brooks Koepka, who is still in position to win his third consecutive PGA Championship. He played beautifully, a final-round 66 good for a tie for second with Johnson.
The match will be contested on the back nine at Sedgefield Country Club at 2 p.m. ET and will be televised as a special presentation of PGA TOUR LIVE and simulcast on Golf Channel and other PGA TOUR media platforms including GOLFTV powered by PGA TOUR to carry the event internationally.
Jason Day saved par from a greenside bunker to get into the clubhouse at 10 under, tied with Matthew Wolff, who shot 65.
Koepka also recognized how tough it is to win a second major. Koepka said he liked his chances entering the final round because he had more major wins than anyone else high on the leaderboard, including Johnson, who only had won one major.
It is Morikawa's third US PGA Tour victory since he made his professional debut in June previous year.
The best shot of his life, the shot that made Collin Morikawa a major champion, was never supposed to happen.
The vanquished Casey - whose long wait for a first career major continues - described Morikawa's epic shot at 16 as "brilliant".
Driver to 7 feet from 294 yards, and then a center-cut eagle putt that gave Morikawa a two-shot lead with two holes to play and CBS a highlight that it will play forever.
Not surprisingly, an elated Morikawa - who is of Japanese and Chinese descent - said he was on "Cloud Nine" after his astonishing exploits, but he admitted something was missing. "As a little kid, watching all these professionals, this is always what I've wanted to do. I think we all know that", he said.
Casey was tied for the lead when he reached the 17th but his hopes of a maiden major championship suddenly slipped away as Morikawa, one group behind, hit the tee shot that led to an eagle and a two-stroke cushion. We certainly shouldn't start thinking that spell of Koepka contending in majors is over, because he's going to be in the mix again when we get to Winged Foot.
World number three Rory McIlroy says his best golf may be behind him after his winless run in the majors continued with a disappointing performance at the US PGA Championship.
Yet he remains one of the most consistent players on the tour and after two wins previous year, he travelled to San Francisco as number 28 in the world.
Instead, he is a runner-up for the first time.
The win at TPC Harding Park is only Morikawa's third professional victory, and his inexperience lifting silverware was evident as the lid of the Wanamaker Trophy fell off as he hoisted it above his head for the cameras.