The 69-year-old, Chicago-area, multibillionaire college dropout admitted in U.S. District Court he didn't report $24.4 million in income from 1999 to 2007 and failed to pay $5.6 million in taxes, the Chicago Tribune reported.
Under his plea agreement, Warner faces 46 to 57 months in prison when he is sentenced Jan. 15. His attorney said he also would pay $53.5 million in civil penalties for not disclosing his foreign accounts.
"When I signed those returns, I knew those moneys were missing," Warner said in court. "It was not accurate. I apologize for my conduct.
"I made a mistake. I'm pleading guilty because I am guilty."
Warner's net worth is estimated at $2.6 billion, and his attorney told the Tribune the businessman has paid more than $1 billion in federal taxes during his lifetime.