Five days after capturing his first world title in the 100m, Lyles pulled away down the straight to win in 19.52 seconds, just shy of his world-leading time of 19.47.
"Winning double golds was one of the things on my list (of goals)," Lyles said. "I wanted to show I am different. Today I came out and showed it. I am double champion."
US teammate Erriyon Knighton (19), bronze medallist at last year's worlds, upgraded to silver in 19.75, while Letsile Tebogo (20) of Botswana, second in the 100m final, took bronze in 19.81.
"If I can keep going the same way, maybe next year the gold will be mine," Knighton said. "If I can join these guys every year, that is a whole lot of motivation. It does not matter what the colour of this medal is."
Lyles, nails manicured with gold stars, served notice of big things a night earlier when he led the semi-finals field by two tenths of a second despite a freak golf cart accident on his way to the stadium that delayed his semi-final heat by 30 minutes.
Tebogo added: "My performance in Budapest means a lot to me, to the country and to the continent, because it is not about me, it is about the people.
"I believe I am a role model for young people in Botswana because there have not been many sprinters from my country."
GOLD STARS
Lyles, nails manicured with gold stars, served notice of big things the previous night when he led the semi-finals field by two tenths of a second despite a freak golf cart accident on his way to the stadium that delayed his heat by 30 minutes.
Jamaican Andrew Hudson (27) had medical treatment for glass in his eye from the crash and was given a lane in the final despite finishing fifth in his semi. Hudson finished eighth out of nine.
"It's been a tough two days for me," Hudson said. "I didn't get back from hospital till 2 a.m. last night. Today it doesn't really matter what lane they gave me, they could have put me on the grass, I was going to run.
"The eye is doing better but I am going to see a specialist in the United States when I get back to check the cornea isn't scratched."
Britain's Zharnel Hughes (28), bronze medallist in the 100m, was fourth, while American Kenny Bednarek (24) came fifth.
Olympic champion Andre De Grasse (26) of Canada, who has been dealing with a nagging foot injury this season, was sixth.
Lyles and Bolt are the only three-time winners over 200m in history, while the illustrious group of 100/200 champions also includes Americans Tyson Gay, Justin Gatlin and Maurice Greene.
"Just reached a level that I believe only five other male sprinters have ever achieved, just became the double world champion," Lyles said. "What a World Championships."