Leishman, who led by five shots heading into the final round at Midland Country Club, finished at 21-under-par 267 and tied the all-time Nationwide Tour record for largest winning margin.
Keoke Cotner (68) ended 11 shots back in second place at 10-under 278.
With a birdie at the second hole Sunday, and an eagle at the fourth, Leishman's first win was all but secured before the 24-year-old Australian even made the turn.
'I was determined to get off to a good start,' he said. 'When I made the eagle at four I was able to play more conservative.'
He dropped a shot with a bogey at the fifth, but got it right back with a birdie at the sixth.
Leishman made two more bogeys down the stretch ' at the 15th and 18th holes ' but by then his lead had grown to double-digits, secured by three birdies between the ninth and 14th holes.
Leishman's first win came at an important time. With just three events remaining on the schedule, he moved from 56th place on the money list to 19th.
Players inside the top 25 at the end of the season will earn PGA Tour cards for next year.
'Winning again would be good,' a relieved Leishman joked.
Aron Price (72) finished alone in third place at 9-under 279, with Greg Chalmers (68), Darron Stiles (70), Won Joon Lee (71) and Spencer Levin (72) sharing fourth at 8-under 280.
Only seven more players ended within 15 shots of Leishman