Merrick led by as many as two on the back nine in regulation and shared the lead for much of the day as a tightly bunched leaderboard made for a tense finish.
Now in his seventh season as a US PGA Tour member, he finally lifted the Northern Trust Open title, and a $1.188 million first prize, with a par at the second hole of sudden death -- Riviera's demanding par-four 10th.
"Growing up here in LA, I just wanted to come to this tournament," said Merrick, who still lives in nearby Long Beach where he grew up and attended the University of California at Los Angeles. "Now to win it, this is a dream come true."
Merrick was one of three players who started the day three shots off the lead held by Bill Haas and he had four birdies and two bogeys in a final-round 69 for 11-under 273.
"I've been in this position a couple of times," Merrick said of chasing the overnight lead. "My coach today just said be tough and patient and that's what I tried to do.
"I hit some really bad shots out there on the back nine and just kind of hung in there. Unless you're Tiger Woods or Phil Mickelson, they win all the time, it's really had to win a golf tournament. I'm just going to cherish this. And, gosh, it's so much fun."
Beljan, who claimed his first tour title last November when he shrugged off a panic attack to win at the Disney World resort, drained an 18-foot birdie putt at the 72nd hole to get to 273 and seize his place in the playoff. The American's four-under par 67 matched the best score of the day.
After both parred the opening playoff hole -- the 480-yard par-four 18th where Merrick made a clutch six-footer -- they returned to 10. Merrick reached the green in two, while Beljan drove into the rough and hit his approach to the fringe 71 feet from the pin.
Merrick's birdie attempt left him some two feet from the cup, and he tapped in for par. Beljan, who had putted up to five feet, then saw his par attempt slip past the cup.
Sweden's Fredrick Jacobson (69), Haas (73) and 2011 Masters champion Charl Schwartzel (70) finished the day one shot back on 274.
Merrick and Jacobson, playing together in the group behind Beljan, had arrived at the par-five 17th tied for the lead on 11-under, and Jacobson looked to have a good chance to move ahead when he stuck his third shot nine feet from the pin after Merrick had found a fairway bunker.
But the American managed to save par, while Jacobson couldn't convert his birdie attempt.
Both Jacobson and Merrick missed the green at 18. Merrick saved par but Jacobson capped his round with a bogey when he missed a five-foot putt.
"You want me to touch that one, only that one? I cannot speak about something else?" Jacobson said when asked about 18. "The last putt wasn't very good."
South Africa's Schwartzel missed a golden opportunity to join the group at 11-under when he failed to convert a six-foot birdie attempt at 17.
"I hit some good putts at 16, 17, 18," said Schwartzel, who parred the last eight holes. "The greens were getting really bouncy and they were fast, so it was hard to really hit aggressive putts ... I came close."
Leading final-round scores on Sunday in the $6.6 million US PGA Tour Northern Trust Open (x-won at 2nd playoff hole. USA unless noted; par-71):
273 - x-John Merrick 68-66-70-69, Charlie Beljan 67-71-68-67
274 - Fredrik Jacobson (SWE) 68-65-72-69, Charl Schwartzel (RSA) 69-67-68-70, Bill Haas 70-67-64-73
275 - Josh Teater 70-68-68-69, Webb Simpson 70-66-68-71
276 - Sang-moon Bae (KOR) 68-65-76-67, Hunter Mahan 70-69-68-69
278 - Greg Chalmers (AUS) 69-69-73-67, Ted Potter 71-67-71-69, Adam Scott (AUS) 71-67-70-70
279 - Ernie Els (RSA) 70-68-73-68, Jim Furyk 68-72-69-70, Sergio Garcia (ESP) 65-73-69-72
280 - John Rollins 69-65-77-69, Seung-yul Noh (KOR) 70-70-71-69, Jimmy Walker 70-70-71-69, Keegan Bradley 71-70-69-70, Luke Donald (ENG) 69-66-70-75
281 - Cameron Tringale 73-69-71-68, Bryce Molder 74-65-74-68, Luke Guthrie 69-71-71-70, Phil Mickelson 71-67-72-71, Lee Westwood (ENG) 68-68-74-71, Graham DeLaet (CAN) 72-68-69-72
282 - Brian Davis (ENG) 70-69-73-70, Bob Estes 68-72-72-70, Kevin Streelman 73-69-72-68, Stewart Cink 71-72-67-72, Greg Owen (ENG) 69-71-70-72, Ryan Moore 70-67-71-74