Houston should have done better in the third minute as they had a four on two opportunity with Will Bruin making a great run down the left but Seattle scrambled well to save.
Brad Davis should have had the opener in the ninth minute as he broke through the Seattle defense after challenging for the ball on the edge of the box and worked through in to space but shot wide.
Dempsey had his first notable impact in the 16th minute as he controlled and sent the ball to Lamar Neagle who shot just wide, leading to an incredible Houston counter that saw them with numbers in the box allowing Davis time to lay up Giles Barnes for the opener.
Houston's good passage continued in the 22nd minute as an incredible strike from 30 yards from Barnes sealed his brace with a contender for goal of the week.
"I knew as soon as I hit it that it was on target," said Barnes. "The only question was if the keeper would get a hand on it and thankfully he didn't"
The dominance continued as Houston continued working and a block fell for Ricardo Clark who was able to then feed Oscar Boniek Garcia who was inches away from making it three.
Dempsey was finally able to get a shot away in the 33rd minute as Neagle was able to put him in but Tally Hall was quick off his line and saved well.
"I felt like maybe I could have done better tonight," said Dempsey after the match. "I had one good chance that I should have put away but all these guys are mid-season and this is my first full 90 in a while."
Houston were left fuming in the 43rd minute after a foul was called back when Giles Barnes could have played advantage with a good opportunity to score but their anger was sated somewhat when Leonardo Gonzalez was shown a yellow card.
Seattle started the second half well with plenty of possession but too often bad decisions and bad finishing cost the Sounders dearly.
Dynamo thought they'd made it three in the 58th minute as Jermaine Taylor headed home from a free-kick but was denied by an offside flag.
Just as it looked like Houston were wrestling back possession a free-kick from Marc Burch whose cross eventually found Shalrie Joseph whose header was deflected in to the net.
Houston got back their two goal buffer in the 74th minute just when it seemed the match would get away as Will Bruin put away the rebound after Brad Davis' initial shot came back off the cross bar.
It really wasn't Seattle's day in the 81st minute when a loose ball in the box started to roll towards goal with Tally Hall stranded only for the ball to bounce off the post and in to the arms of a relieved Hall.
Seattle pushed for a goal late on even bringing on former Scorpions loanee Eriq Zavaleta but it just wasn't to be as Houston held on for the win.