The Dynamo hosts Pachuca next Wednesday at Robertson Stadium in a rematch of their game two weeks ago in Mexico won by the host, 2-0.
Mullan set up goals for Corey Ashe and Stuart Holden after Eddie Robinson capitalized on a goalkeeper's error to open the scoring, and Cam Weaver put away a feed from Ashe to double Houston's lead early in the second half. Holden added Houston's fifth goal in stoppage time after he and Abe Thompson broke in behind the Arabe Unido defense.
Arabe Unido's goal, by Orlando Rodriguez, came with the visitor down to 10 men and temporarily halved to 2-1, but Houston stormed right down the field from the ensuing kickoff and restored the two-goal lead when Ashe tucked away Mullan's deflected cross at the far post.
Robinson scored with ridiculous ease in the 12th minute. Stationed at the far post for a corner kick, he flicked a loose ball into the net after keeper Carlos Bejarano lost the grip on Brad Davis' swerving serve. Waves of Dynamo attacks failed to produce a second goal before halftime, but in the 49th minute Ashe ran onto a ball near the goal line and served a cross to the far post that Weaver headed home from close range.
A second caution to Omar Camargo removed him from the match in the 56th minute yet Arabe Unido got a goal back when Rodriguez ran onto a chip from Alejandro Velez and evaded defender Bobby Boswell, who had replaced Robinson a few minutes earlier, to steer the ball past goalie Tally Hall.
Right from the kickoff, Mullan got free down the right flank and centered a low ball that glanced off a defender's foot and Ashe, unmarked at the far post, shot low past Bejarano. Both goals were recorded as scored in the 69th minute.
Five minutes later, Mullan again found plenty of space on the right flank and this time lofted a ball that Holden, a halftime replacement for Davis, drove into the net with a diving header.
Bejarano stretched full-length in the 86th minute to turn away a penalty kick by Ricardo Clark, who had stepped up to the spot after Brian Ching was fouled. Yet Houston got a fifth goal anyway two minutes into stoppage time when Ching and Thompson beat the offside trap and broke in alone on Bejarano, who had no chance to save Ching's tap-in from Thompson's unselfish square pass.
In Group C play on Tuesday, Kendall Waston scored with a header in the third minute of stoppage time to earn Saprissa a 1-1 draw with the USL's Puerto Rico Islanders. The tie all but ended quarterfinal hopes for the Islanders, a surprise semifinalist last season.
MLS's Columbus Crew, which won at Saprissa last week, is in second place of Group C and hosts Cruz Azul (first-place by a point) on Wednesday (8 pm ET, Fox Soccer Channel & Telefutura).
Concacaf Champions League Group A
Sept. 22 in Houston
Houston 5, Arabe Unido 1.
Goals: Robinson 12, Weaver (Ashe) 49, Ashe (Mullan) 69, Holden (Mullan, Ching) 74, Holden (Thompson) 92+; Rodriguez (Velez) 69
Houston - Hall; Barrett, Hainault, Robinson (Boswell, 64), Waibel, Mullan, Davis (Holden, 46), Clark, Ashe, Weaver (Ching, 70, Thompson.
Arabe Unido - Bejarano; Lewin, Lopez (Carroll, 53), Velez, Aguirre, Santamaria, Justavino, A. Rodriguez (Cooper, 46), Camargo, Mosquera (Medina, 71), O. Rodriguez.
Red card: Camargo 56.
Referee: Paul Delgadillo