Braun, a 19-year-old making his third start on NASCAR's Nationwide Series, posted a fast lap of 102.756 mph in his Ford to edge 48-year-old Scott Pruett. Defending series champion Carl Edwards was third.
"I'm really proud of that whole team," Braun said. "We ran through a whole bunch of changes. ... Shows how much work those guys have put in."
Braun, of Ovalo, Texas, had five years of road course experience before joining Roush Fenway Racing. He earned one pole and two second-place finishes in the Rolex Grand Am Daytona Prototype Series.
Braun's only Nationwide start this year was last month at Nashville Superspeedway, where he finished 15th. This race, on the 2.518-mile road course at Autodromo Hermanos Rodriguez, appeared to suit him.
"I believe he is the guy to beat ... in this race," said Edwards, Braun's teammate with Roush.
Pruett qualified second in his Dodge this year with a lap of 102.651. Pruett took the pole at this event last year in a race Juan Pablo Montoya won.
"We got a little greedy. I just didn't want the car to be tight, and we went too far," Pruett said. "We bumped up the rear tire pressure quite a bit, and unfortunately I couldn't drive the car the way I wanted to."
Edwards won the Nationwide title last year without a single pole. He is second in the point standings this year, although he is still trying for his first victory on the series in 2008.
Edwards will start in the No. 3 spot for the sixth time in nine Nationwide races this year.
"Starting up front is, I think, a big advantage just because of how much can happen here," he said. "I've been working on my qualifying a little bit. We've had great efforts this year."
Marcos Ambrose qualified fourth, and Boris Said was fifth. Kyle Busch, who has won two consecutive Nationwide races, was sixth, followed by Patrick Carpentier. Clint Bowyer, the current series points leader, qualified eighth.
IRL INDY CAR: In Motegi, Japan, the Indy Japan 300 was rescheduled for today because of a wet track. Heavy rain forced the cancellation of Friday's qualifying and the track was still wet on Saturday. Organizers tried to get the track ready but Saturday's race was called off after a two-hour delay.
A persistent wet spot on turn No. 4 was the main cause of the postponement, although crews were working on the track at various places.
Season points leader Helio Castroneves will start on the pole.