"I almost lost my life," the 31-year-old told reporters.
Williams said he had a tumor removed that was attached to his skull:
Williams said he underwent three surgeries following the diagnosis, most recently in June.
The seven-time Pro Bowler held out through the first eight weeks of the 2019 season before reporting to the team Tuesday. However, he failed his physical upon his return because of helmet discomfort.
Williams told reporters he does not blame team owner Daniel Snyder for the way the situation was handled, saying, "It wasn't his fault." However, he said "there's no trust" between him and the organization.
"I was told it was something minor, so I didn't really question it," Williams said of the team's initial assessment. He added, "The diagnosis that they gave me in the beginning, they kind of underestimated it."
According to Kareem Copeland of the Washington Post, Williams "was diagnosed with Dermatofibrosarcoma Protuberans (DFSP), a rare form of cancer. Doctors said it was weeks from moving to his brain when they removed the growth."
Reports surfaced in June that Williams had requested a trade from Washington. Jason La Canfora of CBS Sports reported Williams told teammates he wanted to be traded or released because of his team's handling of a medical situation.
Williams discussed his holdout Thursday:
"Why was I holding out? Like I said, a culmination of things. What I went through health-wise took me away from the game for a minute, anyway. When it pertains to that, just that and the contract status was a thing as well. You know, playing with no guaranteed money, I don't think any, any premier player would want to do that, especially not in the game we play today."
He is signed through 2020 with base salaries of $10.85 million this season and $12.5 million next season, per Spotrac.
Williams cannot be traded until the offseason, as the 2019 NFL trade deadline was Tuesday. According to Copeland, the lineman "just smiles and chuckles" when asked Thursday if he wants to be traded.