E! News said prosecutors agreed to drop six other charges pending against the "Terminator 2: Judgment Day" and "Before and After" star in exchange for his plea Monday.
Furlong was sentenced to 61 days in jail, but was credited for time served, E! said.
The 35-year-old actor was placed on probation for five years, ordered to attend 90 days of a drug-treatment program and instructed to complete 52 weeks of domestic violence counseling.
He also is forbidden to have any contact with his ex-girlfriend.