Mack spent eleven years with the police department of Provo, Utah, and then moved back to Arizona to run for Graham County Sheriff in 1988. While serving as sheriff, he received an invitation to attend the FBI National Academy, and graduated in 1992. In 1994, he was named Elected Official of the Year by the Arizona-New Mexico Coalition of Counties. He was also named the National Rifle Association Law Enforcement Officer of the Year for 1994, and was inducted into the NRA Hall of Fame.
During his tenure as a local sheriff, Mack received national attention for initiating Mack v. United States (later restyled to Printz v. United States), a lawsuit against the federal government which alleged that portions of the Brady Handgun Violence Prevention Act violated the United States Constitution, because they comprised a congressional action that compelled state officers to execute Federal law. These portions were interim provisions until a national instant background check system for gun purchasers could be implemented. In a 5–4 decision, the Supreme Court of the United States ruled that the provisions of the Brady Act in question were, in fact, unconstitutional.