The idea for Léon partly stems from Luc Besson's previous film Nikita (1990), in which Jean Reno played a similar character named Victor; Besson considered Léon to be Victor's 'American cousin', but more human.

Actress-director Maïwenn, who was Besson's wife and whom he met when she was a minor, has claimed that the film was inspired by their own relationship.

Liv Tyler was initially considered for the role of Mathilda before Natalie Portman was cast for her freshness and sensitivity.

Natalie Portman was first turned down by the casting director because of her young age, but she persisted and won the role after performing the scene where her character mourns her brother's death, deeply impressing Luc Besson.

Natalie Portman used her mother's surname for the film to protect her family's privacy, and Léon marked her feature film debut at age twelve.

Gary Oldman agreed to play Norman Stansfield without even reading the script, something he would do again for Besson's next film, The Fifth Element.

Gary Oldman later hinted at some friction with Besson on set, saying the director would tell him exactly how to move, talk and where to stand, though 'not always with the greatest success'.

Samy Naceri, who would later star in Besson-produced Taxi franchise, makes his film debut in Léon in a minor, hooded role as a SWAT team member during the stairwell scene.

Exterior filming took place entirely in New York City from June 1 to July 23, 1993, while interior scenes were shot at the Éclair Studios in Épinay-sur-Seine, France, from July 28 to October 7, 1993.

Some scenes were filmed at the Hotel Chelsea, which houses Léon's apartment, and the Roosevelt Island Tramway was also used as a filming location.

The score was composed by Éric Serra, marking his sixth collaboration with Besson, and was recorded at X-Plorer studios in Paris and Angel Studios in London.

The film features Björk's 'Venus as a Boy', Sting's 'Shape of My Heart' during the end credits, and a Gene Kelly musical number from the film Beau fixe sur New York that Léon watches in a theater.

Natalie Portman sings snippets of Madonna's 'Like a Virgin', 'Singin' in the Rain', and 'Happy Birthday' in one scene of the film.

Mathilda checks into a hotel under the name 'MacGuffin', a nod to Alfred Hitchcock's term for a plot device that drives the story but is otherwise unimportant.

Maïwenn, Besson's wife at the time, has a cameo as the blonde prostitute at the beginning of the film, credited as 'Ouin-Ouin'; she later played the diva in Besson's The Fifth Element.

Luc Besson himself makes a cameo appearance in the film as one of Léon's armed targets holed up in a New York apartment.

Fans have noted similarities between Léon and Hayao Miyazaki's Porco Rosso (1992), whose title character was dubbed in French by Jean Reno, wears similar glasses to Léon, and is likewise the object of affection of a young girl he takes in.

A 1996 extended cut added 26 minutes of footage, including scenes that made the ambiguous relationship between Léon and Mathilda more explicit; some scenes were removed at the request of Natalie Portman's parents, who felt the dynamic was too suggestive.

French screenwriter Franck Gerardi sued Besson, claiming the film plagiarized his script L'Enfance déchirée registered in 1989, but the case was dismissed in 1999 for lack of proven similarities.

Despite receiving seven César Award nominations, including Best Film, Best Director, and Best Actor for Jean Reno, Léon won none of them.

The character Norman Stansfield has been ranked among cinema's greatest villains, placing 43rd on the Online Film Critics Society's 'Top 100 Villains of All Time' list in 2002.