After admission to the Virginia bar, Mullin worked as an assistant prosecutor in
Hampton, Virginia. He became active in the Virginia Bar Association and the Virginia Association of Commonwealth's Attorneys. In November 2016, Mullin defeated Republican Heather Cordasco in a special election to fill the seat once held by Democrat
Monty Mason, who won election to the
Virginia Senate in another special election occasioned by the death of
John Miller. Mullin also defeated Cordasco in the general elections in November 2017 and November 2019. The 93rd district covers parts of
James City County,
York County,
Williamsburg, and
Newport News. In the Virginia House, Mullin serves as vice chair of the Rules Committee, as well as the Labor and Commerce Committee and the Courts of Justice Committee. Mullin served as the chair of a key subcommittee that can conduct hearings for rewriting Virginia's criminal code. On March 12, 2020, he noted progressive reforms passed in that Virginia legislative session, which passed legislation offering additional protections for 14 and 15 year old defendants, as well as ending drivers license suspensions for unpaid fees and fines, raising the felony threshold to $1,000, decriminalizing some marijuana offenses, reforming criminal discovery effective in the summer, expanding deferral options for certain misdemeanors, opening parole opportunities for certain "Fishback" defendants, and allowing jurors to be told about criminal punishment ranges. Mullin sponsored legislation abolishing
Capital punishment in Virginia. In his final year as a legislator, Mullin passed legislation creating roadside historical markers commemorating Virginia locations mentioned in the
Green Book. In June 2024, Mullin became the Chair of the Board for State Navigate, an organization that is working to improve awareness of state legislatures. ==Electoral history==