The 20-year-old carded a closing round of 65 on Sunday to claim a two-shot victory at the Portugal Masters.
The win means Lewis, who was playing in only his third professional tournament, has secured his tour card for next season. He also won more than £375,000.
The unexpected victory also sees Lewis move up to third in the Ryder Cup qualifying race and guarantees him a World Championship debut in Shanghai next month.
However, Lewis will not let his early success go to his head.
"I've got a long way to go. I'm nowhere near the player I want to be," he said.
"I played a couple of events in Australia and Dubai earlier this year. I knew the standard was good, but I knew that I was able to win.
"It was just whether the nerves and the mind was able to carry on."
He added: "I wouldn't have expected this at all at the beginning of the week.
"I got off to a slow start and there are some really good players here, so to come out on top was just unbelievable, especially so early in my career.
"I can't really put it in words at the moment, but if you ask me in a couple of days or a couple of weeks I'll be able to answer in better form.
"I'm just thrilled."
Lewis led The Open in July thanks to a first-round 65 that was the lowest ever score by an amateur in the championship's 151-year history.
Tiger Woods was a previous holder of that record, so now Britain's newest golf star has outdone the former world number one again.
It took Woods five events as a pro to achieve his first success.