Defendant Omari Tahir-Garrett, 56, responded to the verdict by maintaining his innocence and accusing police of lying.
Four police officials who witnessed the assault testified at his trial in King County Superior Court.
Tahir-Garrett, convicted of second-degree assault, was scheduled for sentencing Friday. The crime carries a penalty of three to nine months in jail, and jurors found the assault involved a deadly weapon, which adds another 12 months. Prosecutors will recommend a penalty of 15 to 21 months, said spokesman Dan Donohoe.
The conviction likely will take Tahir-Garrett out of the running for a state Senate seat. He was challenging state Sen. Adam Kline, D-Seattle.
He testified in his own defense before the case went to the jury Tuesday.
Tahir-Garrett, a Central Area activist also known as James Cordell Garrett, testified that he used a megaphone to heckle Schell, who was speaking at a community festival on July 7, 2001.
He said he passed the bullhorn to someone else, then tried to take a photograph and was swarmed by police.
"When I raised the camera, blue came from everywhere," Tahir-Garrett said. "The next thing I know, I was on the ground."
Under cross-examination from Deputy Prosecutor Dan Soukup, he denied knowing who hit the mayor.
"I didn't hit the mayor. I didn't see who hit the mayor," Tahir-Garrett said.
He also testified he receives disability payments for "delayed-stress syndrome due to racism" and described himself as "a refugee of the African slave trade."
He said he barely knew Margie McClure, 50, a self-employed house cleaner who testified Monday that she hit the mayor in a fit of rage, probably with her fist, although she was uncertain about that detail.
In closing arguments, Soukup emphasized that five other witnesses, including police officials and Schell's wife, identified Tahir-Garrett as the attacker. Schell testified that he was blindsided by the attack.
Defense lawyer Eric Weston said Tahir-Garrett was the victim of a police vendetta.
"The state really wants to convict Omari Tahir-Garrett. Who claims they saw this? A police officer, a police officer, a police officer," Weston said.
Tahir-Garrett, who represented himself in the first trial, said he did not know McClure well enough to locate her to testify in May. She said she appeared Monday after learning that her testimony was sought by the defense.
Jurors who were unable to reach a verdict at his first trial in May - they voted 10-2 for conviction - said dissenters on the jury had cited "negative experiences with police."
In a telephone interview Wednesday with Northwest Cable News, Schell expressed horror at the notion that a juror would be dubious about a witness' testimony "just because they're a police officer - or any other job they might have."
Schell expressed satisfaction at the verdict, saying "people need to be held accountable for their actions."
He said of his injuries: "I'm fortunate it wasn't worse than it was. So was Mr. Omari, for that matter."