Production notes

On review aggregator website Rotten Tomatoes, the film has a rating of 81%, based on 191 reviews, with an average rating of 7.3/10. The site's critical consensus states: "Charlize Theron gives a searing, deglamorized performance as real life serial killer Aileen Wuornos in Monster, an intense, disquieting portrait of a profoundly damaged soul." On Metacritic, the film has a score of 74 out of 100, based on 40 reviews, indicating "generally favorable reviews". Monster received highly positive reviews from critics; most gave overwhelmingly high praise to Theron's performance as a mentally unstable woman – Wuornos had antisocial personality disorder and borderline personality disorder. For the role, Theron gained 30 pounds (14 kg), shaved her eyebrows, and wore prosthetic teeth. Critics called her performance, and her makeup, a "transformation". Film critic Roger Ebert named Monster "the best film of the year", gave it four stars out of four, and noted that Theron's role is "one of the greatest performances in the history of the cinema":

Observe the way Theron controls her eyes in the film; there is not a flicker of inattention, as she urgently communicates what she is feeling and thinking [...] Aileen's body language is frightening and fascinating. She doesn't know how to occupy her body. Watch Theron as she goes through a repertory of little arm straightenings and body adjustments and head tosses and hair touchings, as she nervously tries to shake out her nervousness and look at ease. Observe her smoking technique; she handles her cigarettes with the self-conscious bravado of a 13-year-old trying to impress a kid. And note that there is only one moment in the movie where she seems relaxed and at peace with herself. In 2009, Ebert named it the third-best film of the decade. Ricci's performance also drew some praise, but was not without criticism. In his review for the film, Ebert praised her performance, stating "Christina Ricci finds the right note for Selby Wall – so correct some critics have mistaken it for bad acting, when in fact it is sublime acting in its portrayal of a bad actor. She plays Selby as clueless, dim, in over her head, picking up cues from moment to moment, cobbling her behavior out of notions borrowed from bad movies, old songs, and barroom romances". In 2025, it was one of the films voted for the "Readers' Choice" edition of The New York Times' list of "The 100 Best Movies of the 21st Century," finishing at number 195. However, several people who knew Wuornos criticized the movie for portraying her as a victim and her victims as villains.

In 2005, a reference to Monster appeared in the series Arrested Development. Charlize Theron plays the role of Rita in the series, and in the episode ″The Ocean Walker″, a frame from Monster appears on the screen with the clarification that this is a photo of Rita a year ago before the plastic surgery. In 2014, on Saturday Night Live, Charlize Theron made a self-reference to her role of Aileen Wuornos. In the sketch Pet Rescue Commercial, Kate McKinnon asked her to play a cat lady, whose image and behavior are based on Wuornos from Monster. In 2018, comedian Willam released his third album with the song "Aileen" and the music video for the song, which are dedicated to Wuornos and this film.