The plot centers on Léon (Reno), a professional hitman who reluctantly takes in twelve-year-old Mathilda Lando (Portman) after her family is murdered by corrupt Drug Enforcement Administration (DEA) agent Norman Stansfield (Oldman).

Léon is an Italian-American hitman (or "cleaner", as he refers to himself) working for a mafioso named "Old Tony" in the Little Italy neighborhood of New York City.

Mathilda's abusive father attracts the ire of corrupt DEA agents, who have been paying him to stash cocaine in his apartment.

After they discover that he has been stealing from their stash, DEA agents invade the apartment, led by their drug-addicted boss, Norman Stansfield.

When she returns, Mathilda realizes what has happened just in time to continue down the hall to Léon's apartment; he hesitantly gives her shelter.

At first, Léon is unsettled by her presence and considers killing her in her sleep, but he eventually trains Mathilda and shows her how to use various weapons.

Later, while Mathilda returns home from grocery shopping, an NYPD ESU team sent by Stansfield captures her and infiltrates Léon's apartment.

He tells her that he loves her and to meet him at Tony's place in an hour, moments before the ESU team fires a grenade into the apartment, wounding Léon and creating chaos.

The site's critics consensus reads, "Pivoting on the unusual relationship between seasoned hitman and his 12-year-old apprentice—a breakout turn by young Natalie Portman—Luc Besson's Léon is a stylish and oddly affecting thriller.

He said, "Oozing style, wit and confidence from every sprocket, and offering a dizzyingly, fresh perspective on the Big Apple that only Besson could bring, this is, in a word, wonderful".

[19] Mark Deming at AllMovie awarded the film four stars out of five, describing it as "As visually stylish as it is graphically violent", and featuring "a strong performance from Jean Reno, a striking debut by Natalie Portman, and a love-it-or-hate-it, over-the-top turn by Gary Oldman".

[20] Richard Schickel of Time magazine lauded the film, writing, "This is a Cuisinart of a movie, mixing familiar yet disparate ingredients, making something odd, possibly distasteful, undeniably arresting out of them".

Even in a finale of extravagant violence, it manages to be maudlin ... Mr. Oldman expresses most of the film's sadism as well as many of its misguidedly poetic sentiments.

"[23] In the 2013 book, Poseur: A Memoir of Downtown New York City in the '90s, Marc Spitz wrote that the film is "considered a cult classic".

[31] In 2022, UK rapper Knucks released his song "Leon the Professional" as part of his Alpha Place album.

[32] The film has been critically re-examined in the wake of the "#MeToo" movement (French: #BalanceTonPorc or "expose your pig") after sexual assault allegations were levied against Luc Besson in 2018.

[37] In 2011, director Olivier Megaton told reporters that he and Besson used the script for Mathilda as the basis for Colombiana, a film about a young 'cleaner' played by Zoe Saldaña.

[39] According to Besson, this is the version he wanted to release; however, the extra scenes tested poorly with Los Angeles preview audiences.