Williams had been suffering from serious liver disease, the Anchorage (Alaska) Daily News reported. He died Tuesday at 58.
Both prosecution and defense scheduled Williams as a witness, the prosecution because he was in charge of renovations at Stevens' chalet in Girdwood, Alaska, and the defense because he had talked of his respect and liking for the senator. But the prosecution canceled his testimony and returned him to Alaska, citing concerns about his health, which led to defense arguments that his testimony would have undermined the case against Stevens.
Stevens was convicted of failing to report gifts from Bill Allen, president of Veco Corp., an oil services company and Williams' former employee.
Williams had lived in a trailer in South Anchorage for more than 30 years.
He testified before a grand jury investigating Stevens in 2006. He never hired a lawyer or negotiated immunity from prosecution.
"I've got nothing to hide," he said.