Stewards offered to help Bolt with a wheelchair but the sprint legend opted to limp away and applaud the crowd as he made his way around the London Stadium.
"We're world champions, world champions", Mitchell-Blake added through tears of happiness.
The top three from each of the two heats and next two fastest finishers from the remaining qualify for the final round.
With his home nation, Great Britain and America leading on the final bend, the final 100m would come down to the power of Bolt.
US finished second, with a time of 37.52, while Japan took third (38.04). "We had a cool breeze in the back and I think with that a lot of our muscles were cramping up from having our clothes off a little too early".
"I wasn't sure if I had won or not, I gave it my all but I could see Christian Coleman out of the corner of my eye", said Mitchell-Blake.
"I remember after losing the 100m someone said to me: "Usain don't worry, Muhammad Ali lost his last fight also so don't be stressed about that".
'We kept warming up and waiting, then warming up and waiting. In fact Usain said to me: 'Yohan, I think this is insane.
Omar McLeod, who led out the Jamaican team from the traps, said: "It's heart-wrenching".
"This was his farewell race and we wish him the best and hope he recovers soon".
"It was cold and there was 40 minutes and two medal presentations before our run".
The 30-year-old Bolt, beaten into third in the individual 100m behind Gatlin and his United States team compatriot Christian Coleman, was comforted by teammates after getting to his feet, his face etched in pain.
When asked if he would consider a return to racing, he replied: "I've seen too many people retire and come back just to make it worse or to shame themselves".