In 2017, Hardiman was a finalist to succeed Antonin Scalia as an Associate Justice of the Supreme Court of the United States, alongside the eventual nominee, Neil Gorsuch.

[4][11] After graduation, Hardiman joined the Washington, DC, office of the law firm Skadden, Arps, Slate, Meagher & Flom, where he was an associate in the litigation department from 1989 to 1992.

[14] Hardiman was subsequently nominated to the Third Circuit by President Bush on January 9, 2007, to fill a seat vacated by Judge Richard Lowell Nygaard, who assumed senior status in 2005.

[14] In Florence v. Board of Chosen Freeholders (2010), Hardiman held that a jail policy of strip-searching everyone who is arrested does not violate the prohibition of unreasonable searches and seizures in the Fourth Amendment.

Most of the cases that he has encountered involved disputes surrounding the Antiterrorism and Effective Death Penalty Act, a 1996 US law limiting an inmates' ability to seek habeas corpus relief from their conviction.

In United States v. Fisher (2007), Hardiman ruled that a judge could find facts to enhance a criminal sentence according to the preponderance of the evidence standard.

[22] In Busch v. Marple Newtown School District (2008), Hardiman wrote a dissenting opinion in favor of parents who described themselves as Evangelical Christians and were barred from reading from the Bible during a kindergarten "show and tell" presentation.

Hardiman wrote that "the majority's desire to protect young children from potentially influential speech in the classroom is understandable," but that students cannot be barred from expressing "what is most important" about themselves.

[23] However, in August 2019, Hardiman wrote a majority opinion overturning a lower court decision that required the removal of a Christian cross from the Lehigh County Seal because its inclusion served no secular purpose and a reasonable person would see the use of a religious symbol as endorsement of a religion by the government.

[26] Hardiman questioned a 1977 Supreme Court precedent interpreting Title VII not to require such accommodations when they would impose more than a "de minimis cost" on employers.

The Supreme Court granted review and reversed in an opinion by Justice Samuel Alito, rejecting the "de minimis" standard and remanding the case to the Third Circuit.

[28] He wrote that "the most cogent principle that can be drawn from traditional limitations on the right to keep and bear arms is that dangerous persons likely to use firearms for illicit purposes were not understood to be protected by the Second Amendment.

"[29] In the 2013 case Drake v. Filko, Hardiman filed a dissenting opinion that argued that the New Jersey requirement for gun owners to show a "justifiable need" to carry a handgun was unconstitutional.

"[30][31] In Range v. Attorney General (2023), Hardiman wrote the majority opinion for the en banc Third Circuit holding that the federal law barring firearm possession by felons was unconstitutional under the Second Amendment as applied to a man who had been convicted of making false statements to obtain food stamps.

In Valdiviezo-Galdamez v. Attorney General (2010), Hardiman ruled in favor of a man from Honduras who was seeking asylum in the United States to avoid being recruited into a violent gang.

[38] In Cazun v. Attorney General (2017), Hardiman concurred in the judgment to explain that the Immigration and Nationality Act unambiguously forbids aliens subject to reinstated removal orders from applying for asylum and that the court should have held so without resorting to Chevron deference.

[4][46][better source needed] In 2010 Hardiman received the Georgetown University Law Center's Paul R. Dean Award recognizing distinguished alumni.