Clark showed no emotion Thursday in Denver as the verdict was read, but he leaned back and looked at the ceiling once the jury was dismissed. He gave a small smile to relatives before he left the courtroom in handcuffs.
Clark 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."
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 didn't want to speak to reporters.
The jury deliberated for a day and a half after an 11-day trial, then convicted Clark on all 21 counts that he faced, including murder for Williams' death and attempted murder for the 16 other people who were in a Hummer stretch limo with the cornerback.
"Nothing can ever bring Darrent Williams back or ease the suffering for Rosalind and her grandchildren. But after three long years, it is very gratifying to see closure brought to this case," Broncos owner Pat Bowlen said in a statement released by the team Thursday. "This process has been extremely difficult for the Williams family, his friends and teammates, this community, and the entire Denver Broncos organization.
"I would like to praise the long and hard work of the Denver Police Department and the Denver District Attorney's Office," Bowlen added. "I also want to thank the Denver community for the overwhelming support that both the Williams family and the Broncos have received during this unimaginable tragedy.
"Darrent's legacy will live on for all of us in the Broncos organization, and the outstanding work done each day at the Darrent Williams Memorial Teen Center is a tribute to his impact on this community. Our hearts continue to go out to the entire Williams family."
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 earlier this week.
Clark was angry over an altercation, involving his friends and friends of Williams, 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 during the trial that 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 received 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.
The defense suggested during the trial that Harris had fired into the limo. Harris hasn't been charged in the case.
A written exchange between the jurors and District Judge Christina Habas during deliberations seemed to center on the possibility that someone else was involved in the shooting.
Jurors asked Habas if complicity was enough for a conviction. Habas answered that it was, if prosecutors met their burden of proof -- even if jurors found that someone else committed all or part of a murder.
On Thursday, Habas asked the jurors whether any of them had been texting or e-mailing during the deliberations. The foreman replied in writing, "No one has claimed to have sent any text or e-mail."
It wasn't clear why Habas raised the question. After the verdict was read, court officials said jurors didn't want to comment.
Williams was 24 and in his second season with the Broncos when he died. He already was a starter and had four interceptions in the 2006 season, second-best on the team. He was tied for third in tackles with 86.
"The guy had an excellent future ahead of him, and to see it cut short senselessly by violence, it's just really sad," said Nick Ferguson, a former Broncos safety who played with Williams.
"As elated as I am, as happy as I am over the conviction, it won't bring Darrent back to his mom or to his kids," Ferguson added. "But I do know, after all this time, this means a lot for his family. Maybe now, Darrent can rest in peace."
Williams was a native of Fort Worth, Texas, and a star cornerback at O.D. Wyatt High School there. He played at Oklahoma State, where he was a 2003 All-Big 12 Conference 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.
The Associated Press contributed to this report.