Clark showed no emotion as the verdict was read, but leaned back and looked at the ceiling once the jury was dismissed. He gave a small smile to relatives before he was taken from the courtroom in handcuffs.
He faces life in prison at his April 30 sentencing hearing.
Williams' mother, Rosalind, wept as she left the courtroom.
"We'll never know what happened that night," she said later. "This is a start, to clean up the streets here and hopefully everywhere else."
Broncos owner Pat Bowlen issued a statement saying, "After three long years, it is very gratifying to see closure brought to this case."
There was no immediate word on a possible appeal, but defense attorney Darren Cantor told three sobbing people in the courtroom, "Try to breathe, OK? That's what appeals are for."
Cantor told reporters that Clark's family was upset and had no comment.
The jury deliberated for a day and a half after an 11-day trial, and convicted Clark on all 21 counts he faced, including murder for Williams' death and attempted murder for the 16 others who were in a Hummer stretch limo with Williams.
Court officials said jurors didn't want to comment on the verdict.
Security was tight throughout the trial, and 13 armed sheriff's deputies stood in the courtroom as the verdict was read.
Clark declined to testify, citing threats to himself and his family. Cantor said gang members had threatened to turn Clark into "Swiss cheese" if he said anything in court.
Williams was killed at about 2 a.m. on New Year's Day 2007. Prosecutors said Clark fired the fatal shots from an SUV that pulled up beside the rented limo.
"It was this man, who indiscriminately, with universal maliciousness ... took it upon himself to unload his .40-caliber handgun into that limousine full of innocent people," Chief Deputy District Attorney Timothy Twining said in his closing argument.
Clark was angry over an altercation involving friends of Williams and friends of Clark that occurred in a nightclub just before the shooting, prosecutors said. A member of Williams' group had sprayed champagne on New Year's partiers.
Defense attorney Abraham Hutt said Clark wasn't even in the SUV during the shooting.
"This is what this is about: Willie Clark is a scapegoat," Hutt told jurors.
Hutt tried to undercut the credibility of five prosecution witnesses who got shorter prison time in other cases in exchange for testifying. Hutt said the five saw their sentences reduced by a combined 188 years.
Hutt said the prosecution's star witness, Daniel "Ponytail" Harris, faced a life sentence for a drug charge but will be released within two years. Harris testified that he saw Clark fire the shots.
Williams was 24 and in his second season with the Broncos. He was already a starter and had four interceptions that season, second-best on the team. He was tied for third in tackles with 86.
He was a native of Fort Worth, Texas, and was a star cornerback at O.D. Wyatt High School there. He played at Oklahoma State, where he was a 2003 All-Big 12 selection.
The Broncos made him their second-round pick, 56th overall, in the 2005 draft.
In December, Williams had said he wanted to return to Fort Worth in the 2007 offseason to talk to kids about staying out of gangs.