Before his appointment to the District Court, Judge Kennelly worked as an attorney in private practice, representing individuals and corporations in complex civil cases and in criminal cases in trial and appellate courts.

In 1996, he was a recipient of the American Bar Association’s Pro Bono Publico award, a Public Interest Law Initiative citation for distinguished public service, and the Mexican-American Legal Defense and Education Fund’s Community / Public Service award. Judge Kennelly has been a Fellow of the American College of Trial Lawyers since 1999, before his appointment to the bench. Since his appointment to the court, Judge Kennelly has served as the Seventh Circuit's representative on the Information Technology Committee of the Judicial Conference of the United States and as a member of the Information Technology Advisory Committee of the Administrative Office of the U.S. Courts.

In 2019, he was appointed by Chief Justice John Roberts to a seven-year term on the Judicial Panel for Multidistrict Litigation. Judge Kennelly also serves as the vice chair of the Seventh Circuit Criminal Jury Instruction Committee and as a member of the Seventh Circuit Civil Jury Instruction Committee, and he chairs a committee of district judges and lawyers that drafted and periodically updates the local rules for patent infringement cases in the Northern District of Illinois. He has served as president of the Federal Bar Association's Chicago Chapter, the Richard Linn American Inn of Court, and the Lawyers Club of Chicago. In 2021, he was awarded the American Inns of Court Professionalism Award for the Seventh Circuit. Judge Kennelly has spoken extensively regarding various topics, including complex litigation, civil procedure, intellectual property law, employment law, consumer law, and criminal law and procedure.