U.S. Attorney Rod Rosenstein (right) calls Johnson's crimes a "pay-to-play" scheme.
Johnson pleaded guilty to two federal counts of extortion and evidence and witness tampering in a widespread corruption investigation by the U.S. government.
Each count carries a maximum sentence of 20 years. However, the judge still has to decide how many years Johnson will get for his alleged crime.
The plea agreement includes a forfeiture of any money and property acquired through illegal means.
Johnson was indicted in February on charges of extortion, bribery, witness and evidence tampering and conspiracy. Included in his charging documents were transcripts of tapped phone calls in which Johnson instructed his wife to dispose of a check and cash in their home.
Johnson had pleaded not guilty and vowed to fight the charges.
The indictment charges Johnson with accepting cash, campaign contributions and other gifts in exchange for helping developers secure millions of dollars in federal grant money.
His wife, Leslie, a member of the Prince George's County Council, also faces charges in the case. On the day she and her husband were arrested in November, she was found with $79,600 in her underwear.