Léon Montana (Jean Reno) is an Italian hitman (or "cleaner", as he refers to himself) living a solitary life in New York City's Little Italy. His work comes from a mafioso named Tony (Danny Aiello). Léon spends his idle time engaging in calisthenics, nurturing a houseplant, and watching old films.
One day, Léon sees Mathilda Lando (Natalie Portman), a lonely twelve-year-old girl. Mathilda lives with her dysfunctional family in an apartment down the hall. Her abusive father and self-absorbed stepmother have not noticed that Mathilda stopped attending class at her school for troubled girls. Mathilda's father (Michael Badalucco) attracts the ire of corrupt DEA agents, who have been paying him to stash cocaine in his apartment. After they discover he has been cutting the cocaine to keep for himself, DEA agents storm the building, led by sharply dressed drug addict Norman Stansfield (Gary Oldman). During the raid, Stansfield quickly becomes unhinged and murders Mathilda's entire family while she is out shopping for groceries. When Mathilda returns, she realizes what has happened just in time to continue down the hall, where she desperately rings her neighbour's door. A hesitant Léon gives her shelter.
Mathilda quickly discovers that Léon is a hitman. She begs him to take care of her and to teach her his skills, as she wants to avenge the murder of her four-year-old brother. At first Léon is unsettled by her presence, but he eventually trains Mathilda and shows her how to use various weapons. In exchange, she runs his errands, cleans his apartment, and teaches him how to read. In time, the pair form a close bond. Mathilda often tells Léon she is in love with him, but he refuses to reciprocate.
One day when Léon heads out for an apparent assignment, Mathilda fills a bag with guns from Léon's collection and sets out to kill Stansfield. She bluffs her way into the DEA office by posing as a delivery girl, only to be ambushed by Stansfield in a bathroom. Mathilda learns from Stansfield that Léon killed one of the corrupt DEA agents in Chinatown that morning. Léon, after discovering her plan in a note left for him, rescues Mathilda, shooting two more of Stansfield's men in the process. An enraged Stansfield confronts Tony, who is interrogated for Léon's whereabouts.
When Mathilda returns home from grocery shopping, an NYPD ESU team sent by Stansfield captures her and attempts to infiltrate Léon's apartment. Léon ambushes the ESU team and rescues Mathilda. Léon creates a quick escape for Mathilda by smashing a hole in an air shaft; he then reassures her, tells her that he loves her, and thanks her for giving him "a taste for life", moments before the police blow up the apartment. In the chaos that follows, Léon sneaks out of the building disguised as a wounded ESU officer; he goes unnoticed save for Stansfield, who follows him and shoots him in the back. As he is dying, Léon places an object in Stansfield's hands that he says is "from Mathilda"; Stansfield discovers that it is a grenade pin. He then opens Léon's vest to find a cluster of active grenades, which detonates and kills them both.
Mathilda goes to Tony, as Léon had instructed her to do before he died. Tony tells Mathilda he had been instructed by Léon to give his money to her if anything happened to him; he offers to hold it and provide the money on an allowance basis. Mathilda returns to school and meets the headmistress, who readmits her after Mathilda reveals what had happened to her. She then walks onto a field near the school to plant Léon's houseplant, as she had told Léon he should, to "give it roots".
Léon: The Professional is to some extent an expansion of an idea in Besson's earlier 1990 film, La Femme Nikita (in some countries Nikita). In La Femme Nikita Jean Reno plays a similar character named Victor. Besson described Léon as "Now maybe Jean is playing the American cousin of Victor. This time he's more human."
While most of the interior footage was shot in France, the rest of the film was shot on location in New York. The final scene at the school was filmed at the Stevens Institute of Technology in Hoboken, New Jersey.
A soundtrack for the film was released in October 1994. It was commercially successful in Japan, being certified gold for 100,000 copies shipped in December 1999.
Léon: The Professional received favorable reviews from critics. The film holds a 71% positive aggregate rating based on 59 critical reviews on Rotten Tomatoes. The site's consensus states, "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". At Metacritic, the film received an average score of 64 based on 12 reviews, indicating "Generally favorable reviews".
Mark Salisbury of Empire magazine awarded the film a full five stars. 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". 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". 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". He praised Oldman's performance as "divinely psychotic".
Roger Ebert awarded the film two-and-a-half stars out of four, writing: "It is a well-directed film, because Besson has a natural gift for plunging into drama with a charged-up visual style. And it is well acted." However, he was not entirely complimentary: "Always at the back of my mind was the troubled thought that there was something wrong about placing a 12-year-old character in the middle of this action. ... In what is essentially an exercise—a slick urban thriller—it seems to exploit the youth of the girl without really dealing with it." The New York Times' Janet Maslin wrote, "The Professional is much too sentimental to sound shockingly amoral in the least. 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."
Léon: The Professional was a commercial success, grossing over $45 million worldwide on a $16 million budget.
Léon: The Professional won the Czech Lion Award for Best Foreign Language Film and the Golden Reel Award for Best Sound Editing – Foreign Feature. The film was also nominated for seven César Awards in 1995, namely Best Film, Best Actor (Jean Reno), Best Director, Best Cinematography, Best Editing, Best Music, and Best Sound.
In the 2013 book, Poseur: A Memoir of Downtown New York City in the '90s, Marc Spitz describes the film as a "cult classic". In 2014, Time Out polled several film critics, directors, actors and stunt actors to list their top action films; Léon: The Professional was listed at No. 42. The character Norman Stansfield has since been named by several publications as one of cinema's greatest villains.
The British band Alt-J released a song about the film, titled "Matilda" [sic]. The first line in the lyrics, "this is from Matilda", refer to Léon's last words to Stansfield, shortly before the grenades detonate and take their lives. The Bollywood film Bichhoo was inspired by Léon: The Professional.
In 2014, Somali-British director Mo Ali made his second directorial feature Montana, a British action film. The film was inspired by both Léon and The Karate Kid.
South Korean comedian Park Myeong-su and singer-songwriter IU released and performed a song inspired by the film, "Leon", for a bi-annual music festival of South Korea's highly popular variety show, Infinite Challenge, in 2015.
It has been claimed that Besson has written the script for a sequel, which director Olivier Megaton was to direct and in which Portman would reprise the Mathilda role. Filming was to be delayed until Portman was a bit older. However, in the meantime, Besson left Gaumont Film Company to start his own movie studio, EuropaCorp. Unhappy at Besson's departure, Gaumont Film Company "has held The Professional rights close to the vest — and will not budge". According to Megaton, the sequel will more than likely never happen.
There is also an extended version of the film, referred to as "international version," "version longue," or "version intégrale". Containing 25 minutes of additional footage, it is sometimes called the "Director's Cut" but Besson refers to the original version as the Director's Cut and the new version as "The Long Version". According to Besson, this is the version he wanted to release, but for the fact that the extra scenes tested poorly with Los Angeles preview audiences. The additional material is found in the film's second act, and it depicts more of the interactions and relationship between Léon and Mathilda, as well as explicitly demonstrating how Mathilda accompanies Léon on several of his hits as "a full co-conspirator", to further her training as a contracted killer.
Léon: Version Intégrale was released in France in 1996; in the United States, where the film was originally released as The Professional, it was released on DVD as Léon: The Professional in 2000. Both versions contain the additional footage.