Journeyman Merrick sealed victory with a rock-solid par at the second extra hole, the driveable par-four 10th, after laying up off the tee with an iron and hitting his approach to 18 feet.
Beljan missed the green long to the left after using a driver. He left his chip short and then, after a long-range approach putt from just off the fringe, lipped out with a par putt from five feet.
Merrick and Beljan both parred the first extra hole, the uphill 18th, where Beljan had to sink a nerve-jangling six-footer to keep the play-off alive.
"Growing up here in LA, I just wanted to come to this tournament," Merrick, who was born in Long Beach, said greenside after booking a place at this year's Masters. "Now to win it, this is a dream come true."
Merrick, 30, studied at the nearby University of California, Los Angeles and said earlier this week he had probably played at least 30 rounds at Riviera as a student.
The duo had finished the 72 regulation holes on 11-under-par 273, Merrick closing with a two-under 69 and Beljan sinking an 18-foot birdie putt at the last for a joint best-of-the-day 67.
"To make that putt on 18 and hear the roar, was really special," said Beljan, who won his first PGA Tour title at the Children's Miracle Network Classic in November despite suffering a panic attack midway through the tournament.
"Obviously not the way I wanted to end it, but you know what, you win some, you lose some, and that's how it goes."
Swede Fredrik Jacobson finished one stroke shy of joining the playoff after agonisingly missing a five-foot putt to bogey the par-four 18th.
Jacobson signed off with a 69 to share third place at 10 under with 2011 Masters winner Charl Schwartzel of South Africa (70) and American Bill Haas (73), the overnight leader.
U.S. Open champion Webb Simpson (71) was a further stroke back in a tie for sixth with fellow American Josh Teater, who stormed home with an eagle-birdie finish for a 69.
On another sun-soaked day at Riviera where the challenging par-71 layout ran fast and firm, the scoreboard fluctuated wildly as players repeatedly advanced and then stumbled.
Haas, three ahead overnight, made a roller-coaster start with birdies at the first and third and a bogey at the second moving him to 13 under and two strokes in front of playing partner Simpson.
Though Haas did well to save par at the short sixth, where he sank an 11-foot putt, he bogeyed the seventh and eighth for his lead to be cut to just one shot at 11 under.
He then safely parred the ninth to reach the turn in one-over 36, a stroke in front of Hunter Mahan, Merrick, Jacobson and Simpson.
However, Haas again stumbled with a bogey at the difficult 10th after his tee shot ended up well left of the target and his approach rolled through the green into the back right bunker.
Merrick, who had earlier birdied the 10th, edged a stroke clear when he sank a nine-footer to birdie the par-five 11th, before pumping his right fist in delight.
A bogey by Mahan at the par-four 12th handed Merrick a two-shot advantage but Mahan trimmed it back to one when he drained a 24-foot birdie putt at the par-three 14th.
Moments later, Merrick bogeyed the 13th after ending up in the right rough off the tee and missing the green with his approach to slide back into a share of the lead with Mahan.
Mahan's title bid unravelled as he bogeyed 15 and 17, and Jacobson made a late run with consecutive birdies, almost holing out with his approach at the par-four 15th and sinking a 14-footer at the short 16th to join Merrick at the top.
But Jacobson also faltered with his bogey at the last, leaving the stage clear for Merrick and Beljan to battle it out for the title.
- Sports & Recreation