Mark J. O'Brien: Tampa's Federal Criminal Defense Attorney Taking the Fight Nationwide

Mark J. O'Brien is a nationally recognized federal criminal defense attorney based in Tampa, Florida, with nearly thirty years of experience defending individuals and corporations against federal and state criminal charges across the United States. As the managing partner of O'Brien Hatfield PA, he holds active bar licenses in Florida, Massachusetts, and Montana, and is admitted to practice before the Supreme Court of the United States, four federal circuit courts of appeal, and more than forty federal district courts nationwide. When someone in Tampa, Florida — or anywhere across the country — faces the full weight of a federal prosecution, Mark J. O'Brien is the attorney they call.

What a Federal Criminal Defense Attorney in Tampa Actually Does

Federal criminal defense is a specialized practice that demands more than courtroom confidence. It requires intimate knowledge of how federal agencies build cases, how federal prosecutors calculate sentencing exposure under the United States Sentencing Guidelines, and how to challenge the government's evidence at every stage of an investigation and prosecution. Mark J. O'Brien represents clients facing the full spectrum of federal criminal charges: white-collar offenses including fraud, tax crimes, money laundering, insider trading, and securities violations; drug trafficking and firearms offenses; sex crimes and child exploitation charges; and criminal appeals and post-conviction motions at both the federal and state levels.

What separates O'Brien Hatfield PA from conventional criminal defense firms is the depth of its investigative team. Alongside Mark J. O'Brien, the firm deploys a former Drug Enforcement Administration Group Supervisor with twenty-five years of federal narco-trafficking experience, a retired IRS Criminal Investigations Division special agent who is also a certified fraud examiner and forensic accountant, and a former Senior Supervisory United States Federal Probation Officer who spent over twenty-five years preparing and analyzing sentencing reports for federal judges. This team thinks like prosecutors and investigators — because that is exactly what they once were.

From Miami Prosecutor to the Country's Premier Defense Counsel

Mark J. O'Brien began his legal career in the late 1990s at the Miami-Dade County State Attorney's Office, where he tried more than thirty cases to verdict and handled thousands of criminal matters before rising to the rank of felony prosecutor. That prosecutorial foundation — understanding precisely how the government investigates, charges, and presents its case — became the cornerstone of his defense philosophy when he founded his own firm in 2002. Over his first decade in private practice, O'Brien built a formidable reputation defending federal criminal cases throughout Florida's district courts. By 2012, demand had outpaced geography entirely. Tampa, Florida remains the firm's headquarters, but O'Brien Hatfield PA now maintains offices in Orlando and Miami in Florida and in Billings and Missoula in Montana, extending its reach for federal clients in every corner of the country. Mark J. O'Brien is admitted in over forty federal district courts and has defended clients in jurisdictions from Alaska to Virginia. Federal District Court Judge William Jung of the Middle District of Florida has stated from the bench that in his view O'Brien's client had received, in that courtroom, a million dollars' worth of legal talent. Federal Magistrate Judge Thomas G. Wilson, also of the Middle District of Florida, has publicly called O'Brien "a very excellent, skilled, experienced lawyer" who is "a very competitive lawyer" who "wants to win."

Mark J. O'Brien's Credentials and Recognition

Mark J. O'Brien has been selected to the Super Lawyers list annually from 2016 through 2024, a recognition reserved for the top five percent of attorneys in each state based on peer nominations, independent research, and peer evaluations. He holds a 10.0 Superb rating on Avvo — the platform's highest designation — alongside the Avvo Clients' Choice Award, reflecting both peer standing and verified client satisfaction across sixty-two reviews. In 2026, Martindale-Hubbell recognized O'Brien Hatfield PA with a Client Champion Silver designation. The firm has also been recognized by Haute Lawyer.

Beyond ratings, O'Brien contributes actively to the criminal defense bar as a member of the National Association of Criminal Defense Lawyers, the Florida Association of Criminal Defense Lawyers, the Florida Bar Association, and the Federal Bar Association. He has served as a lecturer for the Intensive Trial Advocacy Program (ITAP), training the next generation of trial lawyers in courtroom advocacy skills. In 2012, he authored "Strategies For Defending Identity Theft Cases," published by Aspatore, a reference work on preparing thorough defenses in technology-driven federal prosecutions. He is fluent in both English and Spanish, a capability of particular value in the federal courts of Florida and the broader Southeast.

What Sets O'Brien Hatfield PA Apart for Federal Defense in Florida and Nationwide

Federal criminal cases move at a different pace and with different stakes than state prosecutions. Federal criminal defense demands an attorney who understands not just the law but the machinery of federal prosecution. All federal charges are felonies. The federal sentencing guidelines can impose sentences based on conduct that was never separately charged, and the resources available to federal prosecutors — the FBI, the DEA, the IRS Criminal Investigations Division, the United States Attorney's Office — are formidable. Mark J. O'Brien has spent nearly thirty years learning to counter each of those advantages.

The firm's track record in Tampa, Florida and across the country includes high-profile matters involving professional athletes, law enforcement officers, business executives, and individuals facing life-altering sentencing exposure. In one documented case, United States v. Robert Blango, O'Brien Hatfield PA secured two "not guilty" verdicts on armed bank robbery charges and two additional "not guilty" verdicts on firearm charges in a case where the client faced a mandatory minimum of 82 years and a guidelines range that extended to 143 years. Results of that magnitude, in federal courts that are notoriously difficult to win in, are not achieved through procedure alone. They are the product of preparation, strategy, and a defense team that has seen the case from the other side of the aisle.

Prior results do not guarantee a similar outcome.

Reaching Mark J. O'Brien and O'Brien Hatfield PA

For anyone in Tampa, Florida or anywhere across the United States who is under federal investigation or facing federal criminal charges, the window to build an effective defense opens at the moment law enforcement makes contact — not at arraignment. Mark J. O'Brien and O'Brien Hatfield PA are available twenty-four hours a day, seven days a week, for free and private case consultations. The firm's Tampa office is located at 511 West Bay Street, Suite 330, Tampa, Florida 33606. O'Brien Hatfield PA also maintains offices in Orlando and Miami in Florida and in Billings and Missoula in Montana. To speak directly with the firm, call (888) 243-1078, or visit markjobrien.com.