Matt Kuchar (70), Camilo Villegas (71) and Troy Matteson (73) tied for third at 12 under, and Chris Tidland (68), Stephen Marino (70) and Bob Estes (70) followed at 11 under.
Mickelson, a week after winning the Players Championship, skipped the tournament. Tiger Woods and many of the world's other top golfers did the same.
"I realize that numbers-wise, world rankings-wise, money list-wise, we didn't have everybody here,'' Johnson said. "At the same time, it doesn't matter. It really, really does not matter. There's so many good players every week. I know that gets redundant ... (but) any of the guys who teed it up this week could have won this golf tournament.''
Imada, who began the day tied with Matteson and holding a three-shot lead over Johnson, held his composure after two-putting from 7 feet for par at No. 17.
Rather than dwell on disappointment, Imada drove the middle of the fairway before his approach shot flew the green and landed right on top of a sprinkler head 35 yards behind the pin.
Imada showed some resolve, chipping within 3 feet and tapping in for a birdie that forced a first career playoff for both players.
"I wouldn't say it was the biggest shot of my career,'' Imada said. "Maybe if I had won, but it didn't really change anything. Second is second.''
Johnson had just four bogeys in the tournament, none on the back nine. He picked a perfect time for his first birdie at the par-4 15th hole, rolling in a 14-footer that tied Imada.
Since missing the cut at last year's PGA Championship, Johnson has four top-10 finishes and two others in the top 25 while earning money in 11 of 12 events. He withdrew from his second tournament, the FBR Open in Scottsdale, Ariz., because of a wrist injury.
Johnson has every reason to make plans to return to Atlanta in September, when East Lake Golf Club hosts the Tour Championship, but he doesn't want to discuss it with the Memorial in two weeks and U.S. Open looming in mid-June.
"I'm not a firm believer in expectations,'' Johnson said. "I think (if) you get caught up in expecting to do this or that, things go astray. Paramount is the fact that it doesn't matter where you play.''