Judge Mulroy served as an Assistant United States Attorney (1973-1976) and was a partner at Jenner & Block (1976-2001). He founded his own firm (2001) and became a partner and Chairman of McGuire Woods' Chicago Commercial Litigation Department.

Prior to going on the bench, he practiced law for 35 years as an active corporate trial lawyer. He tried cases in federal and state courts in 11 states. In addition to patent and trademark matters, Judge Mulroy's extensive litigation experience includes trials involving environmental, insurance, real estate, securities, white collar criminal defense, product liability, contract and antitrust issues. He represented the City of Chicago in the mass tort litigation known as the Chicago Flood Litigation.

Judge Mulroy has an extensive record of pro bono representation of indigents through the Federal Defender Program and Catholic Charities. Judge Mulroy was a Director of the Board of the Legal Assistance Foundation and of the Illinois Bar Foundation, organizations dedicated to the assistance of the disadvantaged.

He established "The Mulroy Award for Excellence in Evidence" at Loyola University of Chicago, School of Law, Written and argued thirty civil and criminal appeals in Seventh Circuit Court of Appeals.

A member of the Department of Justice Antitrust Division's Task Force formed to advise Justice on its relationship with counsel.

He was appointed Special Counsel to the Illinois Attorney Disciplinary Commission to prosecute members of the Commission for ethical violations and annotated the Illinois Rules of Professional Conduct for the Illinois Institute of Continuing Legal Education.