Alonzo Monk also agreed to cooperate with federal prosecutors in their investigation of Blagojevich in exchange for a 24-month sentence.
Prosecutors say Monk took part in a wide-ranging corruption scheme but charged him with a single count of trying to shake down a racetrack owner for a $100,000 campaign contribution.
He entered his plea Tuesday before U.S. District Judge James Zagel as part of an agreement that his lawyer and federal prosecutors have been negotiating for months.
The 51-year-old Monk was Blagojevich's law school roommate and became his chief of staff in his first term as governor. He later became a lobbyist.