Arizona's fourth victory in a row, coupled with San Diego's second consecutive home loss to Colorado, put the Diamondbacks 2½ games ahead of the second-place Padres with seven to play.
"I think we're feeling pretty good right now where we're at," said Brandon Webb, who earned his career-best 17th victory. "I think we're playing with a lot of confidence. If we continue what we're doing, we're going to be all right."
Young and fellow rookie Mark Reynolds homered for the second night in a row.
Young's leadoff homer was his ninth of the season, tied for fifth-most in major league history. His 32 home runs are the most by a rookie in the majors this season.
"I mean, that's quite a lift, right off the bat, hitting the first pitch almost out of the stadium," manager Bob Melvin said. "And Reynolds showed a little about his strength, off the front foot, a ball that was almost out on the ground and he gets it out by a fairly decent margin."
The Dodgers extended their losing streak to a season-worst seven games and were officially eliminated from the division race. Los Angeles is mathematically in, but realistically out, of wild-card contention.
Wells (8-9), pitching on three days' rest, gave up five runs and nine hits in 4 2/3 innings for his first loss in six starts for Los Angeles since being released by the Padres. In his last two outings, the 44-year-old left-hander has allowed 10 runs and 17 hits in 9 2/3 innings.
Wells said he was "trying to stop the bleeding."
"We're too good of team to be getting beat like this," he said. "Right now there's no cylinders whatsoever."
The short rest, Wells said, was not the problem.
"I felt great, I felt strong, I had a great bullpen," he said. "But there's nothing you can do when you're behind in the count. They're out there hitting the ball everywhere."
Manager Grady Little said Wells did what the Dodgers wanted him to do after they signed him.
"He was ready for it," Little said of Saturday's outing. "He has been good, but he just wasn't quite enough. We needed him to hit a couple of homers, too."
Webb (17-10) blanked the Dodgers for six innings before giving up a two-run double to Tony Abreu in the seventh. The Diamondbacks ace allowed two runs and eight hits in 6 1/3 innings to improve to 3-0 in his last four starts. He struck out six and walked two.
The Diamondbacks led from the first pitch, when Young homered 468 feet well into the upper deck in left field.
"That was a bomb," Arizona's Chris Snyder said. "Just the first pitch, that got the ball rolling. We just kept rolling from there."
Arizona made it 2-0 two outs later when Snyder doubled home Eric Byrnes from second.
Stephen Drew, 1-for-11 in the first five games of the homestand, tripled with one out in the second, barely sliding in ahead of the throw. Webb singled to right for his third RBI of the season, and it was 3-0.
Reynolds' two-out solo shot off Wells in the third put Arizona up 4-0. In the fifth, Byrnes singled, stole second and third -- his 46th and 47th steals of the season -- then scored on Conor Jackson's sacrifice fly to put Arizona ahead 5-0.
In the third, the Dodgers' James Loney fouled a ball off the plate. The ball bounced hard into the left side of his head, which was not protected by his batting helmet. He flopped to the ground in pain but stayed in the game.
The crowd of 47,673 was the fourth-largest, and perhaps the loudest, for Arizona this season.
"I haven't felt anything like that since back in the day -- '01 and '02," said Melvin, who was bench coach for Arizona's 2001 World Series champions. "You kind of had that feeling, 'I've been here before, I've felt this before.'"
The Diamondbacks made several big defensive plays in their 50th home win of the season.
With runners at first and second, Byrnes made a leaping grab over his head of Mike Lieberthal's line drive to deep left field to save at least a run in the second. Second baseman Augie Ojeda made a running grab of Wells' grounder up the middle in the fourth, then twirled and while falling backward threw the slow-running pitcher out.
- Against the Dodgers, Reynolds has four home runs in 10 games and Young five in 17 games.
- Young's nine leadoff homers are the most in the majors.
- Los Angeles has to finish 3-4 to have a winning season.
- Joe Beimeil made his 80th appearance for the Dodgers, tying Tom Martin (2003) for most by a lefty in franchise history.