Lyles, third in his defence of the 100m title on Sunday, held four fingers in the air after crossing the line as he now matches Jamaican superstar Usain Bolt, who won four in a row from 2009-15.
As Lyles celebrated, Bednarek, who finished a painful fourth in the 100m, looked distraught as he now has two world and two Olympic silvers in the event.
Bryan Levell took bronze in a personal best 19.64, edging Botswana’s Olympic champion Letsile Tebogo to win Jamaica’s first medal in the event since Bolt’s sprint double in 2015.
Lyles had a belated start to the season as he struggled with injury and said he was trying to cram in races to get sharp for Tokyo.
He was not quite there for the 100, where he was beaten by Jamaican duo Oblique Seville and Kishane Thompson, but showed with his season-leading 19.51 in Thursday’s 200m semis that he was approaching his best.
Always the showman, Lyles arrived sporting a new look, with his shaggy hair bleached and bouncing. When his name was announced on the start line he let out a guttural roar to the sky.
“I wanted it pretty bad,” said Lyles, who won bronze in the same stadium in the no-fans 2021 Olympics.
Femke Bol takes gold in the women’s 400 metres hurdles
“One, because I didn’t win the 100, then two, I wanted to rewrite a memory here in Tokyo which just wasn’t as good as it could be, and it wasn’t because of the colour of the metal, it was more because of the state of my mindset, not having the crowd. It’s just such a blessing to have everybody come out here and you can just feel the energy.
“It’s everything you could ever want from a crowd.
“Being patient was the most important thing tonight. I knew some of the guys were going to tighten up but I just stayed relaxed and got the job done.
“This is amazing and such a joyous moment I am going to keep with me forever. Now I want to win the gold medal in the relay and I can’t wait for 2027 to become the only man to win five 200 metres titles.”
Femke Bol, meanwhile, put the seal on a fantastic season by retaining her world 400 metres hurdles title in emphatic style in a world-leading 51.54 seconds.
Jasmine Jones of the US finished like a train to take silver in a personal best of 52.08, while Emma Zapletalova got a surprise bronze in a Slovakian national record of 53.00.
Bol won eight races in a row through the season and started her Tokyo trip by anchoring the Netherlands to silver in the mixed 4x400m relay. In the absence of Olympic champion and world record-holder Sydney McLaughlin-Levrone, who won the 400 flat on Thursday, Bol was red-hot favourite and duly delivered a textbook performance.
American Melissa Jefferson-Wooden became the fourth woman to complete the sprint double at the World Championships with victory in the 200 metres in 21.68 seconds.
The 24-year-old American ran the fastest time of the year to cap a dominant season with another gold medal to add to the one she won in the 100m on Sunday.
Britain’s Amy Hunt was a distant second to earn her first individual global medal with silver in 22.14, while bronze went to double defending champion Shericka Jackson of Jamaica in 22.18.