León Cosgrove

John R. Byrne

John R. Byrne is a partner at LEÓN COSGROVE LLP where he focuses his practice on commercial litigation and appellate matters.

Mr. Byrne specializes in the litigation of complex commercial transactions and consumer fraud matters on behalf of both plaintiffs and defendants.

Prior to joining León Cosgrove, he was an Assistant United States Attorney for the Southern District of Florida. During his time at the U.S. Attorney’s Office, Mr. Byrne charged over 60 cases, including numerous fraud cases. He took 16 of these cases to trial, serving as first chair ten times and second chair six times. During his time as a prosecutor, he developed an extensive knowledge of federal law, including federal statutory law, the Federal Rules of Evidence, and the Federal Rules of Criminal Procedure.

Mr. Byrne also has extensive federal appellate experience. In addition to having argued a case before the U.S. Court of Appeals for the Eleventh Circuit, he served as the law clerk to Chief Judge Jeffrey R. Howard of the U.S. Court of Appeals for the First Circuit.

Prior to his time as an Assistant U.S. Attorney, Mr. Byrne practiced with Ropes & Gray LLP, where he focused on internal investigations, regulatory enforcement matters, and complex commercial disputes.


Obtained a $66 million award for alternative energy client following a three week arbitration hearing in Chicago. Proved that wind turbine manufacturer supplied client with defective wind turbine blades.

Represented a Latin American former head of state in extradition proceedings, obtained an order releasing said individual on bond and dealt with numerous issues of first impression.

Represented energy company in fraud action in federal court, resulting in a $10 million settlement.

Represented prominent former NCAA basketball coach in connection with federal investigation arising out of Southern District of New York. No charges filed.

Represented former State Department official in connection with high-profile federal lawsuit in the Southern District of Florida, successfully obtaining protective order governing his testimony.

Community Involvement

Federal Bar Association, South Florida Chapter, Board of Directors

Riverside House, Chair of Young Professionals Board