The 29-year-old from Denver was the joint overnight leader with compatriot Rickie Fowler at the Los Angeles Country Club and carded a level par 70 with his final round to end on 10-under for the tournament.

Northern Ireland's McIlroy, who started the day a shot behind the leading pair, could only total level par as well as his nine-year wait for the fifth major of his career continued.

Fowler slipped up with a five-over-par final round which saw him slide into a share of fifth place on five-under.

Clark had finished no better than 75th in his sixth previous majors and his underdog win came not far from Hollywood.

Last month, he had won the Wells Fargo Championship for his first PGA Tour win

The new world number 13 played a key shot at the eighth when he chipped out of long grass at the third attempt to limit his damage to just a bogey.

Clark also saved par at the ninth with a crucial seven foot putt, following a poor tee shot.

McIlroy had earlier missed a similar putt for birdie and he could not put enough pressure on despite Clark dropping shots at the 15th and 16th to set-up a nervy finish.

Clark will receive $3.6 million (£2.8 million/€3.3 million) for his victory as part of a total prize pool of $20 million (£15.6 million/€18.3 million).

"It hasn't quite hit me yet," said Clark.

"Walking up 18 was pretty emotional, and then finishing. 

"It's been a whirlwind the last five, six weeks. 

"Just so blessed and humbled to be here."

Clark had lost his mother from breast cancer while he was attending Oklahoma State

"I just felt like my mom was watching over me today," he said afterward. 

"I worked so hard and dreamed about this moment for so long. 

"I just felt like it was my time."

McIlroy conceded that the long wait to win a fifth major title was making him increasingly frustrating.

"It is, but at the same time, when I do finally win this next major, it's going to be really, really sweet," he said. 

"I would go through 100 Sundays like this to get my hands on another major championship."