In addition to becoming the oldest winner of the tournament at age 54, Lewis earned an invitation to next year's Masters.
Lewis, who finished second in this event 15 years ago, won the opening two holes at the Shadow Hawk Golf Club in Richmond, Texas, and never trailed. He held a 2-up lead after 18 holes and pushed his advantage to 4-up with five holes to play.
Lewis defeated three-time former champion Nathan Smith to get to the finals.
The winner of the Mid-Amateur Championship is one of six amateurs each year to be invited to the Masters. The other five are the finalists for the U.S. Amateur and the winners of the British Amateur, the Asian Amateur and the U.S. Public Links Championship.