DeChambeau, who began the day eight adrift, fired an eight-under par 63 in the closing round at Rich Harvest Farms in suburban Chicago to finish 54 holes on 13-under 200.
That was good enough to give the American a one-stroke triumph over Australian Marc Leishman and India's Lahiri, who led most of the last round and would have forced a play-off with a closing par.
It made for a bittersweet victory for DeChambeau, captain of the triumphant Crushers team whose lineup includes Lahiri.
"I wanted to be in a play-off with 'Ban," DeChambeau said. "He has worked his butt off all year.
"I couldn't be happier that I won but I feel so bad for 'Ban. I wanted to go into a play-off and battle it out with my brother. I love him to death. He's a grinder."
DeChambeau made six birdies on his back nine and birdied the par-5 second, his penultimate hole, to grab a share of the lead on 13-under with Lahiri, who had stumbled with a bogey at the par-3 15th.
At 18, Lahiri left a long birdie attempt short and sent his par putt left of the hole.
"He had this won but I just luckily snuck in there and took it down," DeChambeau said.
DeChambeau could capture the season title in the Saudi-backed series with a victory in a regular-season finale next month in Saudi Arabia.
The US Open 2020 champion, who won his first LIV title last month at Greenbrier thanks to a final-round 58, answered a bogey at nine with three consecutive birdies, the last a long effort over a ridge.
"I just went on a roll from there," he said.
"I would say (this is) the most consistently good golf of my life. I don't know if it's the top notch but definitely when I shot 58 that was the best golf I've ever played.
"This whole stretch has been pretty sweet for me."
Lahiri, a four-time LIV Golf runner-up, last won in February 2015 with European Tour triumphs at the Malaysian and Indian Opens.
He began the day three adrift but birdied the par-5 second and par-3 fourth to grab the lead at 12-under after bogeys by Colombian Sebastian Munoz at the third and fourth.
He then birdied the par-5 10th to grab a share of the lead and was alone at the top after Munoz made bogeys at 11 and 13, only to fall to DeChambeau's fiery finish.
Five-time major champion Brooks Koepka, this year's PGA Championship winner who will compete for the Americans at next week's Ryder Cup in Italy, closed with a long par putt at the third hole to shoot 71 and finish eight strokes back.
Crushers won the team event by three strokes over the Sergio Garcia-led Fireballs.
The final LIV Golf League tournament of the regular season will be staged October 13th-15th at Jeddah with a team championship the following week at Doral in Miami.
