Helms took an 0-2 fastball from McClung (1-1) out to left, dropping the Brewers to 5-3 in extra-inning decisions.
"It was the right mindset, Mike [Rivera, Brewers catcher] made a good call and the coaching staff has a good gameplan for these hitters," McClung said. "It was what I wanted to throw. It just wasn't where I wanted to throw it."
Matt Lindstrom (1-0) retired two batters with a runner in scoring position in the ninth and earned the win. The struggling Milwaukee offense managed just five hits, though nine others reached base via walk or hit by pitch.
"There were probably four instances where we hit the ball right on the nose that could have changed the whole complexion of the game, but we hit them right at people," Brewers manager Ned Yost said. "That's what happens when you're struggling a little bit to score runs. You just can't find those holes."
Bill Hall, who had entered the top half of the inning as part of a double switch, lofted a home run in the seventh against Taylor Tankersley and tied the game at 2.
At the time, momentum clearly favored Milwaukee, after left-handed reliever Mitch Stetter wormed out of a nightmare in the top of the seventh. After Derrick Turnbow loaded the bases with no outs and the heart of the order due up, Stetter punched out Jeremy Hermida and Josh Willingham before getting a pop-out from Jorge Cantu to keep the Marlins off the board for the frame.
"I knew with Hermida I had the lefty, so my thought process was to get a strikeout and with Willingham as a righty, try to get a ground ball and see if we could turn a double play," Stetter said. "There's a comfort level. I really feel comfortable out there."
Milwaukee scored a run in the first inning without the benefit of a hit, loading the bases with one out on two walks and a hit batsman before Corey Hart's sacrifice fly to left.
Outfielder Willingham had to leap at the last second to make the catch, preventing the Brewers from cashing in even more on the wildness of starter Ricky Nolasco, who still allowed just one run on two hits in five innings.
"We have a good team, so we can't change much," Hart said. "We don't really want to change much. Right now, we're just not getting the break we need."
Brewers starter Manny Parra has yet to pitch through the sixth in five starts this year, leaving after 5 1/3 innings Sunday with two runs and six hits allowed. Parra struck out three and walked three others.
"Today, I was really surprised I threw that many pitches," said Parra, who exited after throwing 99 tosses, 57 for strikes. "I went out there thinking my pitch count was a lot lower than it was. I felt like I was going after them, pitching very aggressively and doing a lot of the things I wanted to do. I didn't know I was going as deep into counts as I was."
Parra did sniff out a double steal in the fifth, with runners at the corners and two outs. Parra threw to first base as Dan Uggla broke for second, and Prince Fielder fired to catcher Rivera to nab Hanley Ramirez, who was streaking for home.
But the Marlins tallied twice in the sixth. Cantu brought home a run with a one-out double, and Helms hit a run-scoring bloop single that chased Parra from the game.
Florida again put its first two runners on in the eighth before Guillermo Mota struck out the side. The two teams combined to leave 25 men on base before Kevin Gregg locked down his fourth save.