"It's their town," said D-backs manager A.J. Hinch. "They can carry us for four days if they want to."
Snyder's three-run home run in the fourth inning broke a 1-1 tie, and Young had three of Arizona's 12 hits and three RBIs.
"It's good to jump on a team that is struggling," said second baseman Kelly Johnson, who went 1-for-5 and earlier on Monday was named the National League Player of the Month for April.
The loss was Houston's seventh straight, and for the second time in three games, the Astros lost by at least eight runs. Felipe Paulino (0-4) took the loss, allowing four runs in six innings.
Valdez went five innings, allowing only a first-inning run. He gave up five hits and three walks while striking out two.
"It's so exciting," said Valdez, who relied a lot on his sinker. "The first two innings, I was nervous. After that, I felt good."
"He earned the win," said Hinch. "I was impressed how he held in there."
Valdez put the D-backs in position to win and he was the beneficiary of the three relievers who did not allow a run in four innings. Esmerling Vasquez relieved Valdez in the sixth with two scoreless innings, followed by Aaron Heilman in the eighth inning and Blaine Boyer in the ninth.
Valdez got off to a shaky start with a 35-pitch first inning. The Astros scored their lone run as Carlos Lee's double scored Michael Bourn.
Bourn opened the first with a walk, Valdez's first big league batter.
"I told myself, 'What am I going to do now?'" said Valdez.
But Valdez settled down, and after a rough first inning, looked like a veteran on the mound.
"His first two innings, you could tell that the game was going quick for him," said Snyder, who has three home runs in his past four games. "Fortunate for him, the action he has on the ball, he's able to go right after guys and work himself right back out of it."
Young, who graduated from Bellaire High School in Houston, drove in a run in the second inning and added a two-run single in the seventh to give the D-backs a 6-1 lead.
"Our offense got going tonight," said Young.
Arizona added three runs in the eighth to put the game safely out of reach.
Mark Reynolds went 2-for-4 with two runs scored.
Snyder played at Houston Spring Woods High School (Roger Clemens' high school) and the University of Houston. He crushed his fourth-inning home run over the railroad tracks in left field, his fifth of the season, on a 96-mph offering from Paulino on a 2-2 count with two outs.
"It was a good at-bat," said Snyder. "The guy throws hard, he challenged me. He's got a good fastball. It's probably his best pitch. I just got good wood on it."
The Astros wasted opportunities in the early innings off Valdez. They left the bases loaded in the second and stranded two runners in the third with the game tied at 1.
Bourn robbed Adam LaRoche of extra bases leading off the sixth. Bourn, who won a Gold Glove Award last season, leaped to snare the ball near the 373-foot mark in right-center in front of the Astros' bullpen.
However, Bourn, a college teammate of Snyder, could only watch the D-backs catcher's home run from his center-field position.
"He caught it good," said Bourn. "He had to be looking for that pitch. The way he swung, that ball got out of here in a hurry.
"All you can do is tip your cap to him. He did what he was supposed to. That was a big hit and put them up by three in the game."