Mannix is a driving force behind the effort to get tougher sentences for criminals. In 1994, he authored Ballot Measure 11, which established mandatory minimum sentences for violent crimes, including murder, manslaughter, serious assault, kidnapping, rape, sodomy, unlawful sexual penetration, sexual abuse, and robbery. This measure has also now been used to set automatic sentencing on accidental deaths due to addiction issues. A companion measure, Measure 10, also passed, amending the Oregon Constitution to provide that any criminal sentence established by a vote of the people cannot be reduced by the legislature except with a two-thirds vote of both houses. A third successful Mannix measure, Measure 17, amended the Oregon Constitution to establish a new section which requires that state prison inmates work full-time in useful work. This also allowed work time to include education courses, counseling, and job training.
