Montero and Young hit the first slams of their careers, both connecting off Brandon Backe. Montero also had a solo homer and double, driving in a career-high five RBI as the Diamondbacks won for the fifth time in six games.
Dunn, acquired on Monday in a trade with Cincinnati, went 2-for-2 and scored twice. He has at least one hit in all five games with his new team.
Though Dunn is stuck on 32 homers, manager Bob Melvin thinks the slugger has helped the others just by being in the lineup.
The numbers support Melvin's notion -- Arizona has at least 10 hits in the five games Dunn has played, including 12 on Saturday night.
"Adam's presence in the lineup takes pressure off a lot of guys, makes our lineup deeper," Melvin said. "So far, so good since he's been here."
Montero had only one home run this season before Saturday night and that came on Aug. 2. He's the backup catcher and didn't know when he'd get another chance to hit one.
"It's something I can't control," Montero said. "Whenever I play, I'll do my best and I'll be ready whenever they put me in."
Young, a Houston native, homered for the second straight night in front of friends and family. Arizona has hit six home runs in two days and Young also points to Dunn as a catalyst.
"When you add somebody like that, it gives the entire offense a push," Young said.
Yusmeiro Petit (2-3) allowed four hits and two runs in seven innings. The 23-year-old Venezuelan righty has made just five starts since getting recalled from Triple-A on June 27, and this was his first start since Aug. 7.
Melvin has also used Petit three times as a reliever since his recall.
"Whatever role you give him, he's ready to go out there and handle it," Melvin said.
Backe (7-12) gave up 11 earned runs for the second time in three outings. He also allowed 11 in 3 1/3 innings against the Cubs in Chicago on Aug. 6.
Ty Wigginton hit an RBI triple and two doubles for the Astros, who've lost eight of the last nine against Arizona.
Montero's slam capped a five-run first for the Diamondbacks. He began the game with only one home run in 132 at-bats this season.
Backe came in with a 7.56 ERA in first innings this season and he ran into immediate trouble by walking Stephen Drew and giving up a single to Young.
"I just didn't have anything," Backe said. "It was the same as Chicago. It's frustrating. A lot of the time, I wasn't missing by much."
Montero led off the fourth with another homer to right. It was his second career two-homer game and his first since he hit a pair against San Diego last Sept. 5.
Dunn walked in the fifth and unexpectedly stole his second base of the season. Reynolds singled to center and Dunn beat the throw home for a 7-1 lead.
Montero barely missed another homer in the sixth, hitting a liner off the right-field wall for a double. Former Astros player Chris Burke singled and Drew walked to load the bases again.
Houston manager Cecil Cooper visited the mound, but stuck with Backe. Young, a Houston native, then launched Backe's 119th pitch into the left-field porch, his 17th homer of the season. Young also homered in Arizona's 12-2 win over the Astros on Friday.
Cooper trudged to the mound again and brought in Geoff Geary to relieve as Astros fans booed and filed out of the ballpark.
"I was just trying to squeeze Brandon through the sixth inning," Cooper said. "I was just trying to get him to get one more guy out, but it didn't happen."
- Arizona has won 14 of its last 21 road games.
- Wigginton extended his hitting streak to 13 games.
- The Diamondbacks have had at least 10 hits in five straight games.
- Drew had a 17-game hitting streak snapped.
- The Astros placed 2B Kazuo Matsui on the 15-day disabled list with an irritated disk in his back and purchased the contract of minor leaguer J.R. House.