Twenty-two-year-old Dwright Doty was found guilty of first-degree murder in 2015, when Tyshawn's small body collapsed on the concrete in the alley, a bullet wound on the head.
"Even at a time when we were almost deafened by the headlines about violence and that has deeply shaken us", Cook County Prosecutor Kim Foxx said after the verdict.
"While we wait for the second verdict, we are pleased with the first result of this case," she said. "We hope that this verdict will bring a degree of closure and peace to the family of Tyshawn Lee that we will never forget."
The other man, Corey Morgan, was tried in the same courtroom, but in front of a separate jury. That body was still thinking when Doty was convicted.
The prosecution said the boy was targeted as a revenge and "executed" after Doty and Morgan suspected that the gang, which included Tyshawn's father, was behind the murder of Morgan's brother. They said Doty was the shooter.
The jury witnessed for nearly three weeks, and Doty thought for several hours.