After being homeschooled for her entire life and having spent the last twelve years in Africa, 16-year-old Cady Heron begins her first day at North Shore High School. She has trouble making friends and is put down by teachers on her first day. She eventually befriends outcasts Janis Ian and Damian Leigh, who explain the school's various cliques, warning her about the "Plastics": the insecure Gretchen Wieners, the dimwitted Karen Smith, and the ruthless queen bee Regina George. Cady is unexpectedly invited to join the Plastics; Janis hopes to use it to collect information on Regina, whom she accuses of "ruining her life". Later that day, Cady becomes infatuated with her classmate Aaron; Gretchen warns her that as Regina's ex-boyfriend, he is off-limits.

Regina discovers Cady's infatuation with Aaron and tells Cady she does not care about their relationship. Despite Janis's insistence that Regina is "evil", Cady comes to enjoy hanging out with the group at the mall and writing insulting remarks about their classmates and teachers in a diary called the "Burn Book". However, at a Halloween house party, instead of talking to Aaron on Cady's behalf, Regina kisses him in front of her and resumes their relationship. Feeling betrayed, Cady fully commits to Janis's plan to ruin Regina's life.

Over the following months, Cady, with Janis and Damian's help, manages to turn Gretchen against Regina and trick her into revealing her secrets, break up Regina's relationship with Aaron (and spend time with him by pretending to need math tutoring), and cause her to gain weight by giving her high-calorie snack bars under the pretense that they are diet food. After violating the Plastics' dress code rules by wearing sweatpants, the only thing that fit her, Regina is banished and Cady becomes the new "queen bee".

With her parents out of town, Cady throws a house party. She drunkenly admits to Aaron that she does not actually need math tutoring; he renounces her as being no better than Regina. Janis angrily confronts Cady for throwing a party instead of attending her art show, and declares that she has become as plastic as she pretended to be. Meanwhile, Regina is enraged to discover Cady's sabotage and pastes her own photo in the Burn Book to frame Cady, Karen, and Gretchen.

Regina distributes photocopies of the book throughout the hallways, inciting chaos. Principal Duvall and math teacher Ms. Norbury gather the female junior students in the gym to talk through their social issues. When Janis reveals her plan against Regina to the students' cheers, Regina furiously storms out and, while confronting an apologetic Cady, she is struck by a school bus, fracturing her spine.

After realizing a comment she wrote in the Burn Book has framed Ms. Norbury as a drug dealer, Cady decides to take full responsibility for the book, becoming a social pariah. To earn extra credit, Cady joins the school Mathletes at the state finals, answering the tiebreaker correctly to win the championship. The team arrives at the Spring Fling dance, where Cady is unexpectedly elected queen. She breaks the plastic tiara and distributes the pieces to others in the crowd, makes peace with those she had wronged, and shares a slow dance and a kiss with Aaron.

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• Lindsay Lohan as Cady Heron, a 16-year-old girl who transfers to a public high school after being homeschooled her whole life in Africa
• Rachel McAdams as Regina George, a rich, popular mean girl and the leader of The Plastics
• Tina Fey as Ms. Sharon Norbury, a math teacher and Mathletes moderator
• Tim Meadows as Mr. Ron Duvall, the principal of North Shore High School
• Amy Poehler as Mrs. George, Regina's mother
• Ana Gasteyer as Mrs. Heron, Cady's well-meaning mother
• Lacey Chabert as Gretchen Wieners, a rich Jewish girl and member of the Plastics, Regina's second-in-command and best friend
• Lizzy Caplan as Janis Ian, Cady and Damian's goth friend and artist
• Daniel Franzese as Damian Leigh, Cady and Janis' openly gay friend
• Neil Flynn as Mr. Heron, Cady's father
• Jonathan Bennett as Aaron Samuels, Regina's ex-boyfriend and Cady's love interest
• Amanda Seyfried as Karen Smith, the dimwitted third member of the Plastics
• Rajiv Surendra as Kevin Gnapoor, the Mathletes president and an amateur rapper who is attracted to Janis
The film's large cast also includes Molly Shanahan and Jonathan Malen as Kristen Hadley and her boyfriend, Ky Pham and Danielle Nguyen as "Cool Asian" students Trang Pak and Sun Jin Dinh, and Daniel DeSanto and Diego Klattenhoff as jocks Jason and Shane Oman, Gretchen and Regina's respective boyfriends. Dwayne Hill plays the school's predatory gym teacher Coach Carr.

Alisha Morrison plays "unfriendly" student Lea Edwards and Jan Caruana plays overweight student Emma Gerber. Wai Choi plays mathlete Tim Pak. Julia Chantrey plays Amber D'Alessio and Jacky Chamberlain and Olympia Lukis appear as disabled students Giselle Sgro and Jessica Lopez. Stefanie Drummond plays offbeat student Bethany Byrd.

Tyson Fennell appears as Cady's childhood love interest Nfume. Alexandra Stapley portrays student Taylor Wedell and Laura DeCarteret plays her mother, while Nicole Crimi portrays Regina's younger sister Kylie and Dan Willmott appears as Regina's father. Sharron Matthews portrays Mr. Duvall's secretary Joan and Clare Preuss plays Marymount Prep student Caroline Krafft.

Tina Fey read Rosalind Wiseman's Queen Bees and Wannabes and called Saturday Night Live producer Lorne Michaels to suggest it could be turned into a film. Michaels contacted Paramount Pictures, who purchased the rights to the book. As the book is nonfiction, Fey wrote the plot from scratch, borrowing elements from her own experience at Upper Darby High School and her impressions of Evanston Township High School and New Trier Township High School from her time living in the Chicago area, upon which the film's fictional "North Shore High School" is based.

Fey named many characters after real-life friends. In a 2014 interview about the movie, she told Entertainment Weekly, "I tried to use real names in writing because it's just easier." Main character Cady Heron was named after Fey's college roommate Cady Garey. Damian was named after Fey's high school friend Damian Holbrook, who went on to become a writer for TV Guide. Minor character Glenn Coco is named after a friend of Fey's older brother; the real Glenn Coco works as a film editor in Los Angeles. Janis Ian was named after singer Janis Ian, who was one of the musical guests on the first Saturday Night Live episode, in which she sang the song "At Seventeen", which can be heard playing in the background when the girls are fighting at Regina's house. The film was originally going to be called "Homeschooled".
Fey, Michaels, and Paramount's chairman Sherry Lansing wanted to cast Lindsay Lohan after seeing her performance in Freaky Friday (2003). Lohan was originally set to play Regina George, but the casting team ended up feeling she was their strongest choice for the role of Cady Heron, and since executives feared the "mean girl" role would alienate her audience, she eventually agreed to play the lead. Rachel McAdams was cast as Regina because Fey felt McAdams being "kind and polite" made her perfect for such an evil-spirited character and Waters felt Lohan was "a little intimidated" by her which made him like their dynamic. McAdams had originally auditioned for the role of Cady but was deemed "a bit too old" for the character. Kristen Stewart, Michelle Trachtenberg and Mae Whitman were considered for the role of Cady opposite Lohan's Regina George early on. According to casting director Marci Liroff, Blake Lively was the top choice to play the role of Karen Smith, but was told by the producers to keep looking. Amanda Seyfried originally auditioned for Regina, and although director Mark Waters thought she was "fantastic", the producers instead suggested her for Karen due to her "spacey and daffy sense of humor". Leighton Meester, Haylie Duff, Kate Mara and Megan Fox were also up for the role of Karen. America Ferrera and Kat Dennings were both heavily favored for the role of Janis Ian. Lizzy Caplan was at first considered too beautiful for that role, for which Waters felt a "Kelly Osbourne-type" was necessary…

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