Mark J. O’Brien

Mark J. O’Brien is a criminal defense lawyer at O’Brien Hatfield with extensive experience in federal and state criminal cases. His practice covers white collar investigations, fraud offenses, drug and gun charges, sex offenses, appeals, and post-conviction matters. He began his career as a prosecutor with the Miami-Dade County State Attorney’s Office. His work has since included complex criminal proceedings in Florida and federal courts across the country.

Mark earned a B.A. from Siena College. He later received his J.D. from Delaware Law School. His legal education prepared him for a career centered on criminal litigation, trial advocacy, and courtroom practice. He entered the profession in 1998 and was admitted to practice in Florida and Massachusetts that year.

Mark moved into criminal defense after his prosecutorial work and developed a nationwide federal practice. He represents individuals and corporations in investigations and prosecutions involving tax fraud, wire fraud, securities fraud, health care fraud, money laundering, bribery, and related offenses. His work extends to federal grand jury investigations, appeals, sentencing issues, and post-conviction proceedings. He has practiced before numerous federal district and appellate courts.

Mark J. O’Brien serves as a managing partner and continues to focus on federal and state criminal defense. He is admitted in Florida, Massachusetts, and Montana, along with the Supreme Court of the United States. His professional memberships include the National Association of Criminal Defense Lawyers, Florida Association of Criminal Defense Lawyers, and Federal Bar Association. His background combines prosecution experience, trial work, appellate practice, and long-term criminal defense representation.

Law Office Affiliations

• Attorney, O’Brien Hatfield
• Member, Florida Bar
• Member, Massachusetts Bar Association
• Member, Montana State Bar Association
• Member, American Association for Justice
• Member, National Association of Criminal Defense Lawyers
• Member, Florida Association of Criminal Defense Lawyers
• Member, Federal Bar Association

Licensed State

• Florida
• Massachusetts
• Montana

Education

• Delaware Law School
  Juris Doctor (JD)
• Siena College
  Bachelor of Arts (BA)

Awards & Badges