He graduated from University of Notre Dame with a Bachelor of Arts degree in 1978 and Harvard Law School with a Juris Doctor in 1981, where he was Executive Director of the Harvard Legal Aid Bureau.

He served as a law clerk for Judge Prentice Marshall of the United States District Court for the Northern District of Illinois from 1982 to 1984.

He reentered private practice in 1984 and served in that capacity until his appointment to the federal bench in 1999.

On January 26, 1999, Bill Clinton nominated Kennelly to be a judge on the United States District Court for the Northern District of Illinois to a seat vacated by Paul Edward Plunkett.

He gained widespread attention when he presided over the case of ACLU v. AT&T in 2006,[2] "a lawsuit filed by the American Civil Liberties Union of Illinois on behalf of author Studs Terkel and other activists who said their constitutional rights were violated because of an NSA program of gathering phone company records."