Judge Hardiman was appointed to the United States Court of Appeals for the Third Circuit on January 9, 2007 and was confirmed by the Senate (95-0) on March 15, 2007. Prior to becoming an appellate judge, Judge Hardiman served as a trial judge on the United States District Court for the Western District of Pennsylvania as of November 1, 2003. In 2008, Chief Justice John Roberts appointed Judge Hardiman to the Information Technology Committee of the Judicial Conference of the United States. Judge Hardiman was appointed Chairman of the IT Committee in 2013, and he served in that capacity until 2020.

Before entering judicial service, Judge Hardiman handled a wide variety of litigation matters in state and federal trial and appellate courts as a partner at Reed Smith LLP (1999-2003), a partner at Titus & McConomy LLP (1996-1999), and as an associate with its predecessor firm, Cindrich & Titus (1992-1996). Judge Hardiman began his legal career as an associate in the Washington, D.C. office of Skadden, Arps, Slate, Meagher & Flom (1990-1992).

A 1987 honors graduate of the University of Notre Dame, Judge Hardiman received his law degree in 1990 from the Georgetown University Law Center, where he served as a Notes and Comments Editor on the Georgetown Law Journal. In 2012, Judge Hardiman was elected as a member of the American Law Institute and was elected to its Council in 2019. He has taught a seminar on Advanced Constitutional Law at Duquesne University School of Law since 2012.

A native of Waltham, Massachusetts, Judge Hardiman has chambers in Pittsburgh, Pennsylvania, where he and his wife Lori reside. They have three grown children.