The 18-year-old rider gained two places in a fighting last lap of a hectic five-lap sprint. It was part two of a race that had been red-flagged earlier after a crash.
Team-mate Efren V�zquez (26, from Spain) added to the Mahindra team's celebrations by claiming two points, finishing 14th.
The sprint race came about after the planned 18-lapper was red-flagged before the required two-thirds distance had been completed. Positions counted only for grid places in the restart, over just five laps of the fine new 5.5-km circuit outside the Texas state capital of Austin.
"I just attacked," he explained.
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V�zquez had also moved up from 15th to 14th for the restart; and held his position.
The all-independent new Mahindra MGP3O has made its mark on the competitive Moto3� class in just two races, six months after it first took to the drawing board. In Qatar the white-and-red racers were seventh and tenth.
Now the squad is looking forward to the Spanish GP at Jerez in two weeks, where the twisty circuit promises much.