[2] Leon received his Bachelor of Arts degree from College of the Holy Cross in 1971, where he played varsity lacrosse[3] and was a classmate of future Supreme Court Justice Clarence Thomas.

"[9] During a hearing on October 23, 2008, Leon commented on the ambiguity of the term "enemy combatant" and criticized Congress and the Supreme Court: "We are here today, much to my dismay, I might add, to deal with a legal question that in my judgment should have been resolved a long time ago.

[11][12] In December 2008, Leon denied Moath Hamza Ahmed al Alawi's habeas corpus petition, finding the enemy combatant's detention was lawful under the Authorization for Use of Military Force of 2001.

[15][16] On December 16, 2013, Leon ruled that the NSA's bulk collection of Americans' telephony records likely violated the Fourth Amendment to the United States Constitution, though he stayed enforcement of his injunction pending appeal to the D.C.

[...] No court has ever recognized a special need sufficient to justify continuous, daily searches of virtually every American citizen without any particularized suspicion.

[...] The Government does not cite a single instance in which analysis of the NSA's bulk metadata collection actually stopped an imminent attack [...] Because of the utter lack of evidence that a terrorist act has ever been prevented because searching the NSA database was faster than other investigative tactics – I have serious doubts about the efficacy of the metadata collection program [...] I have little doubt that the author of our Constitution, James Madison [...] would be aghast.In January 2010, Leon preliminarily enjoined the Food and Drug Administration from blocking the importation of electronic cigarettes.

[19] In 2010 he threw out the charges in an obscenity case against director John Stagliano: "I hope the government will learn a lesson from its experience", calling the Justice Department's prosecution "woefully insufficient".

In a motion for a stay of Leon's order, lawyers for the plaintiffs said if they returned or destroyed the memo, "this critically important evidence of murder will likely be lost forever.

[30] On June 12, 2018, Leon rejected all of Assistant Attorney General Makan Delrahim's claims and refused to block the $85.4 billion merger of AT&T and Time Warner.

In expressing skepticism over the merger, Leon cited opposition from the American Medical Association (AMA), which previously warned that the deal would lead to higher premium and out-of-pocket costs for patients purchasing drugs, as well as reduce the quality of health insurance.

He said the Fourth Amendment against unreasonable searches and seizures "offers little refuge for unenclosed land near one of the country’s external borders" and that environmental review had been properly waived by the DHS.

[33] On July 19, 2019, Leon upheld the Trump administration's health insurance expansion that allowed companies to offer additional plans that do not meet the coverage requirements of the Affordable Care Act.

[36] On December 30, 2019, the subject dismissed Charles M. Kupperman v United States House of Representatives, et al., a case, briefly, at the center of both the Trump–Ukraine scandal and the impeachment proceedings against President Donald Trump which became moot due to the subpoenas being withdrawn.