Niemann carded a final round of even-par 71 to finish 19-under 265, two shots clear of World No.2 Collin Morikawa and Cameron Young.

Australian Adam Scott – who won the event at the famed Los Angeles layout in 2020 and the unofficial 36-hole champion in 2005 – had two eagles in a five-under 66 to share fourth with World No.4 Viktor Hovland at 14-under-par.

Starting Sunday with a three-shot lead, Niemann's margin was down to one when he made a seven-foot birdie on the eighth hole and PGA Tour rookie Cameron Young sailed the green and made bogey.

Niemann chipped in for eagle on the par-5 11th to stretch his lead to five but two back-nine bogeys and a late charge from two-time major winner Morikawa (65) threatened to derail his victory parade.

Two eagles in the final round propelled Adam Scott into a share of fourth. PHOTO: Cliff Hawkins/Getty Images.

However, Morikawa – who earlier had chipped in on the seventh and 10th holes – missed a 10-foot birdie putt on the 18th that would have ramped up the pressure while the hopes of Young (70) faded with a bogey from the bunker on the 16th.

For Niemann, Riviera hasn't had a 23-year-old winner since Phil Rodgers in 1962, and this time the winner pocketed a cool US$2.16 million ($A3m) for his second PGA Tour victory.

Charlie Sifford in 1969 was the last player to go wire-to-wire with Niemann trumping a high-quality field that attracted the world's top-11 players.

After blistering the historic Riviera course to the tune of 16-under-par through the first two rounds, Niemann backed off the throttle to shoot three-under on Saturday before Sunday's controlled final round.

"This weekend took me forever; it felt like a month," Niemann said on the CBS broadcast.

"I am so happy it's finally done.

"I'm really proud of the way we battled with my caddie Gary (Matthews). All of the hard work, there is nothing like this.

"This is amazing."

Justin Thomas shot 70 and finished alone in sixth place at 13-under, while Maverick McNealy and Scottie Scheffler were tied for seventh at 12-under after carding 69s.

"This weekend took me forever; it felt like a month. I am so happy it's finally done." - Joaquin Niemann.

Scott got off to a flying start with an eagle on the opening hole and birdies on the fourth and sixth before his round was stalled by three bogeys in six holes on his back nine.

He offset two of those dropped shots with birdied on the 11th and 14th holes before a long eagle putt on the 17th snared him a US$540,000 ($A750,000) pay day.

"I've got a couple things I'd like to work on before I peg it up next time, but I'm pegging it up with a bit more confidence the next time I play...," the Queenslander said.

"I wouldn't change anything much, but just a little more practice and hopefully it keeps kind of embedding itself into my routines out there and I make a few more putts and a few less bad shots and hopefully be winning soon."

Of Scott's fellow Australians, Matt Jones (68) and Marc Leishman (73) shared 15th in a group at eight-under while Cameron Smith (70, five-under) and Cam Davis (69, three-under) made jumps up the field on the final day.