Calvert R. McMahon

Senior Counsel

  • Office Contact Info
  • Biography

    Calvert McMahon is Senior Counsel in the Oklahoma City office of GRSM and is a member of the Employment Law, Commercial Litigation, Tort & Product Liability, Construction, and Professional Liability Defense practice groups. Calvert previously worked at a private law firm in Tulsa, Oklahoma, where his main focus dealt with medical malpractice defense, construction, insurance defense, and trucking and transportation. He also regularly counseled large healthcare systems on issues pertaining to potential liability exposure and employment law.

    Prior to private practice, he served as assistant general counsel for the Oklahoma Tax Commission and the Office of the Oklahoma Attorney General. Specific to his time at the Oklahoma Tax Commission, he frequently counseled attorneys, state employees, various state agencies, and business owners on issues regarding local, state, and federal tax law and compliance with the same. Additionally, he routinely engaged in negotiations with taxpayers, drafted contracts pertaining to payment agreements, and represented the state of Oklahoma in administrative proceedings before the Oklahoma Tax Commissioners.

    In his capacity as Assistant Attorney General for the Office of the Oklahoma Attorney General, Calvert handled all matters regarding complex tobacco laws. As a heavily regulated industry at both the state and federal level, he frequently communicated with other attorney generals nationwide regarding current litigation, legislation, and legislative trends. Additionally, he advised business owners, outside counsel, accountants, various government agencies, and state personnel on matters regarding tobacco regulations and law. Additionally, he frequently drafted complex contracts concerning multi-million-dollar escrow agreements to ensure that tobacco manufacturers complied with state and federal laws.

    In addition to the aforementioned, he is well-versed in both property law and immigration law, as he has represented clients in matters relating to these two fields. His work experience has allowed him to develop a diverse skill set, which makes him equally suited for litigation, research, compliance, and transactional work.


    • Oklahoma
    • U.S. District Court, Northern and Western Districts of Oklahoma


    • Oklahoma Bar Association

    Community Involvement

    • Head Coach of Daughter’s Soccer team
  • Practice Areas
  • Representative Experience
    • Defended healthcare system and its employees in jury trial, which resulted in a defense verdict on all counts for his client.
  • Education


    J.D., University of Tulsa College of Law, 2014

    B.S., summa cum laude, Finance, Oklahoma State University


    Top Collections Attorney, Oklahoma Tax Commission, 2016