Mexico, set to open World Cup play June 11 against host South Africa, finished its U.S. tour 4-0-2 without allowing a goal.
"The second half we played more open, but that was a mistake because we needed to play more together," Mexico coach Javier Aguirre said. "We gave Angola an advantage because they are so fast. They have a lot of speed, and being open hurt us."
Guardado sent the raucous crowd — the third largest for a soccer game at Reliant Stadium history — into a frenzy when he scored off his own rebound.
Guardado took a pass from Alberto Medina and fired a left-footed shot from 12 yards that was batted out by Luis Joao Lama. Guardado stayed with the play and got the rebound 7 yards out in the center and nailed a right-footed shot into the left corner.
Mexico missed three chances in the first half, including a shot by Alberto Medina that went off the corner of the crossbar and a shot from Guardado from short of midfield that was batted over the crossbar.
"We have a poor offense," Aguirre said. "We have to work on it. Our defense is working well. They are doing a good job. I'm not worried about the defense. My only concern is the offense."
Angola didn't have a shot on goal until the 80th minute, but Guillermo Ochoa made a diving save to stop Maeco Antonio Rasca. Angola had two more chances late, but shots by Amandio Felipe de Costa and Hermenegildo Bartolomeu went high.
"I'm not worried about the result," Angola coach Herve Renard said. "We only had three weeks to prepare, but the team is made up of local players, and they played well."
Mexico will play Chile on Sunday in Mexico City on Sunday.