Mikolas turned in two perfect innings in his eighth Major League appearance.
San Diego pitcher Clayton Richard scored the winning run on a two-out single by Will Venable. Richard was forced to pinch-hit with two out in the 13th after San Diego manager Bud Black used all of his position players. Richard scampered all the way home from first after Los Angeles outfielder Howie Kendrick booted the ball.
"My first win, being in an extra-inning game and a walk off, it was really exciting," Mikolas said. "Kind of an unexpected way to get it. I was just pumped."
Mikolas is 1-1 with a 5.40 ERA. He has worked two scoreless innings in each of his last two appearances and has yet to give up a hit to a left-handed batter.
The 23-year-old was brought up from Double-A San Antonio on May 4.
Mikolas was one of the first players to greet Richard at the plate after he scored the game-winning run.
"It was crazy," Mikolas said. "I'm going to get the lineup card and probably frame it."