Nationwide Federal Criminal Defense

Last updated on December 18, 2025

O’Brien Hatfield is a criminal defense law firm with a primary focus on nationwide federal criminal defense. The law firm is led by Federal Criminal Defense Attorney Mark J. O’Brien, a nationally recognized federal criminal defense attorney with nearly 30 years of experience in federal criminal courts throughout the country.

Federal Criminal Defense Attorney Mark J. O’Brien has represented clients in federal criminal courts from Florida to Alaska, Arizona to New York, Massachusetts to California, and in nearly every state in between. He can practice federal criminal defense anywhere in the country.

Federal Criminal Defense Attorney Mark J. O’Brien

Mark J. O’Brien, Esquire is a nationally recognized Federal Criminal Defense Attorney with over 25 years’ experience in criminal courts all throughout the country. After beginning his professional career as a Miami, Florida prosecutor in the late 1990’s, Mr. O’Brien organized his own law firm in 2002 based upon the professional lessons he learned to that point in his life with the goal defending those accused of federal crimes with a mix of old school values and new school ideas.

During his first decade in private practice from 2002 to 2012, Mr. O’Brien defended individuals and corporations charged with federal crimes throughout federal criminal courts in Florida. From 2012 to the present, Mr. O’Brien has accepted the defense of clients in federal criminal courts throughout the entire country, case after case, success after success, in federal court after federal court, all throughout the country, has resulted in a growing legacy. To put it simply, Mr. O’Brien wins and, as a result, he is a lawyer in demand. Mr. O’Brien grew up in New York, attended law school near Philadelphia, Pennsylvania, and started his career in Miami, Florida. There is nothing he has not seen, done, or dealt with in and/or out of a criminal courtroom. New York raised, Philly tough, and Miami tested, Mr. O’Brien brings his 25 years of federal criminal court experience to those accused of federal crimes in federal criminal courts throughout the nation. Mr. O’Brien has the resources, both in his team and in his experience and his training, to tackle your federal criminal case and accomplish the only objective that matters: Win.

Admitted to Practice Law:

State of Florida
Commonwealth of Massachusetts
State of Montana
Supreme Court of the State of Florida
Massachusetts Supreme Judicial Court
Montana Supreme Court
Supreme Court of the United States

Past Federal Cases In the Following Federal Jurisdictions:

United States Court of Appeals for the First Circuit
United States Court of Appeals for the Second Circuit
United States Court of Appeals for the Ninth Circuit
United States Court of Appeals for the Eleventh Circuit
United States District Court for the Northern District of Alabama
United States District Court for the Eastern District of Arkansas
United States District Court for the District of Arizona
United States District Court for the Central District of California
United States District Court for Connecticut
United States District Court for the Southern District of Florida
United States District Court for the Middle District of Florida
United States District Court for the Northern District of Florida
United States District Court for the Southern District of Georgia
United States District Court for the Middle District of Georgia
United States District Court for the Northern District of Georgia
United States District Court for the Southern District of Illinois
United States District Court for the Southern District of Iowa
United States District Court for the Northern District of Iowa
United States District Court for the Southern District of Indiana
United States District Court for the District of Kansas
United States District Court for the Eastern District of Kentucky
United States District Court for Massachusetts
United States District Court for the Western District of Michigan
United States District Court for the Southern District of Mississippi
United States District Court for the Northern District of Mississippi
United States District Court for the Western District of Missouri
United States District Court for Montana
United States District Court for the Southern District of New York
United States District Court for the Northern District of New York
United States District Court for the Western District of New York
United States District Court for the Middle District of North Carolina
United States District Court for the Eastern District of North Carolina
United States District Court for North Dakota
United States District Court for Northern District of Oklahoma
United States District Court for Rhode Island
United States District Court for the Eastern District of Texas
United States District Court for the Western District of Texas
United States District Court for the Northern District of Texas
United States District Court for the Eastern District of Virginia
United States District Court for the District of Columbia (Washington DC)
United States District Court for the Eastern District of Washington
United States District Court for the Western District of Washington
United States District Court for the Southern District of West Virginia

Federal Criminal Defense Team

In addition, Federal Criminal Defense Attorney Mark J. O’Brien assembled a criminal defense team to aid their criminal defense lawyers. Like how the law firm lawyers shape their strategy and approach to figure out what the federal prosecutor is going to do before he or she does it, Federal Criminal Defense Attorney Mark J. O’Brien assembled former federal agents and state law enforcement professionals to figure out what federal law enforcement will do before they do it.

Federal Criminal Defense Attorney Mark J. O’Brien’s team includes a former Drug Enforcement Agency (DEA) Federal Special Agent who specializes in drug trafficking and gun crimes, a former Internal Revenue Service Criminal Investigation Division (IRS CID) Federal Special Agent who focuses on white collar and fraud crimes, a former United States Probation Officer who concentrates on sentencing guideline issues and mitigation efforts, and a former Tampa Police Department Detective who assists in all defense investigations.

Federal Criminal Defense Attorney Mark J. O’Brien’s team also includes a support staff of experienced paralegals and dedicated legal assistants. All these team members work only for the law firm and directly for you. As a result, there will never be a conflict of interest.

Federal Criminal Defense

Facing federal criminal charges is a serious, high-stakes matter that requires immediate and strategic representation. Federal cases differ significantly from state prosecutions, involving complex laws, specialized procedures, and often more severe sentencing guidelines. Having an experienced federal criminal defense attorney on your side can mean the difference between protecting your rights and facing harsh penalties.

Why Federal Cases Are Different

Federal criminal charges are investigated and prosecuted by federal agencies such as the FBI, DEA, IRS, ATF, and Homeland Security. These agencies often conduct extensive investigations before filing charges, giving them a substantial advantage from the outset. The rules governing federal prosecutions, including the Federal Rules of Criminal Procedure and the Federal Sentencing Guidelines, require defense attorneys to have deep knowledge and a focused approach.

Experience in Federal Criminal Law

Our team handles cases prosecuted by federal agencies such as the FBI, DEA, IRS, ATF, and Department of Homeland Security. Federal charges often result from extended investigations and can include offenses such as:

• White collar crimes: fraud, embezzlement, insider trading, securities violations
• Drug offenses: trafficking, conspiracy, manufacturing
• Firearms and weapons offenses
• Cybercrimes and identity theft
• Tax evasion, healthcare fraud, and financial crimes
• Immigration-related offenses
• Public corruption and bribery

We also defend federal sex crimes, conspiracy crimes, grand jury investigations, and pre-file criminal investigations.

Our Approach

Federal Criminal Defense Attorney Mark J. O’Brien understands that each case is unique and demands a tailored defense. Our legal team will:

• Analyze every aspect of the government’s investigation
• Challenge improper search warrants and seizure of evidence
• Negotiate strategically with federal prosecutors
• Present compelling arguments in court to protect your rights
• Seek dismissal or reduction of charges whenever possible

Protecting Your Rights from Day One

The earlier a federal defense attorney is involved, the better the chances of influencing the outcome. Federal prosecutors move quickly, and any statements or actions taken before you obtain legal counsel can be used against you. If you believe you are under investigation – or have already been charged – contact us immediately to begin your defense.

Contact Us

Federal Criminal Defense Attorney Mark J. O’Brien defends individuals and corporations accused of or charged with federal criminal offenses in every state in the country. Because he is licensed to practice law in multiple jurisdictions, Mr. O’Brien is able to defend federal criminal cases anywhere without be licensed to practice law in any one particular state. Mark’s office is often an airplane with WiFi access. And because he is often not from the place he defends his clients, Mark is unburdened by a local attorney’s concern of making waves with the local judiciary and/or prosecutors.

Federal Criminal Defense Attorney Mark J. O’Brien is committed to providing aggressive, skilled representation in federal court. We stand ready to protect your freedom, reputation, and future. Reach out today to Mark’s legal assistant Jeanette for a free confidential consultation.