Garciaparra had his first multihomer game in nearly six years and Matt Kemp also homered for the Dodgers, who won their second straight to pull into a first-place tie with the Diamondbacks.
"It's still early," Garciaparra said. "We have to focus on just winning ballgames."
Loney's home run off Doug Slaten (0-3) was the Dodgers' third solo shot over the final five innings and the club's season-high fourth of the game.
"You never know what to expect coming out of the break," said Dodgers manager Joe Torre.
Cory Wade (1-1) pitched the 10th for the Dodgers and one-time closer Jonathan Broxton, Los Angeles' eighth pitcher, came on in the 11th for his first save in six opportunities. He's filling in for injured closer Takashi Saito.
"They're still going to watch me," Broxton said. "That's why they're not calling me the closer."
Garciaparra finished 3-for-4 with two runs scored and three RBIs. He hit two homers in a game for the first time since August 4, 2002, for the Boston Red Sox against Texas.
Young tripled into the right-field corner and scored in the third inning to put Arizona ahead 7-5, but the Los Angeles bullpen held the Diamondbacks scoreless on six hits over the final eight innings.
"When it gets to point where it does tonight, the 10th or 11th inning, the game can go to either team," Young said. "It's just a matter of who gets the big hit and tonight Loney did."
Garciaparra hit his second home run to lead off the sixth inning against reliever Yusmeiro Petit and pull the Dodgers to 7-6. Kemp then hit a one-out homer off Leo Rosales in the seventh to tie the game at 7.
"We got the bats going early, they got the bats going early," Kemp said. "It was a back and forth battle."
The teams combined for seven runs on nine hits, two walks and three errors in the first inning as the Diamondbacks took a 4-3 lead. Andre Ethier and Russell Martin had RBI doubles for Los Angeles while Tracy and Mark Reynolds had run-scoring singles for Arizona.
"Both teams swung the bats well the first couple innings and looked sloopy at times, but then tightened it up considerably," Arizona manager Bob Melvin said.
The Diamondbacks added two more runs in the second on an RBI double by Tracy and a run-scoring wild pitch by Dodgers starter Hiroki Kuroda to make it 6-3.
Garciaparra pulled the Dodgers to 6-5 in the top of the third with his first home run, a 416-foot shot into a bowl of food in the pool area beyond the fence in right-center field off Diamondbacks starter Doug Davis.
Kuroda lasted a season-worst two innings, allowing six runs -- five earned -- on eight hits with a walk and a strikeout.
Davis fared little better, giving up five runs -- four earned -- on five hits with a walk and three strikeouts over a season-low three innings.
Dodgers CF Andruw Jones left the game after the top of the fifth with what Torre described as a stomach flu. Jones, who earlier missed 38 games following arthroscopic knee surgery, had struck out twice and struggled to chase down a pair of doubles off the wall. ... The Diamondbacks placed right fielder Justin Upton on the 15-day disabled list, retroactive to July 9, with a strained left oblique. Upton is hitting .242 in 83 games with 11 home runs and 31 RBIs. ... Arizona reacquired 1B Tony Clark from San Diego in exchange for a minor league pitcher. Clark, who spent the past three years with the Diamondbacks before signing with the Padres as a free agent, struck out as a pinch-hitter in the ninth. ... Dodgers hitting coach Don Mattingly made his official dugout debut Friday night, giving Los Angeles a pair of league MVPs on the coaching staff for the first time in franchise history. Torre was the NL MVP in 1971 while Mattingly won the AL award in 1985.