The film starts with Rodrick waking up his brother Greg, saying it is time for school. However, Rodrick tricks Greg, as it is already night. After the title sequence, Greg attends his first day at middle school and discovers the ups and downs, such as the missing stall doors in the boys' bathroom and the difficulties of obtaining a seat during lunch break. During P.E. lessons, he and his best friend Rowley escape from a game of Gladiator and learn about the Cheese Touch - a rotten piece of cheese on the basketball court that makes anyone who touches it an outcast. Greg also meets Angie, a girl who isolates herself from the other girls to survive. Greg states his intention of becoming the most popular student in school.
At the end of the day, Rowley unintentionally embarrasses both boys by asking Greg if he wants to come over and "play". Greg looks through Rodrick's yearbook at home and Rodrick catches them, threatening to kill Greg. The next day, Greg signs up for wrestling to become popular but suffers humiliating losses against both Fregley, a weird outcast, and Patty Farrell, Greg's arch-enemy from elementary school. On Halloween, the two boys encounter teenagers while trick or treating, and they are drenched with water. When Greg threatens to call the cops, the teenagers chase them to his Grandma's house. Some time later on, Greg makes it an effort to make Rowley popular by changing his style of clothing and looks, and how he wears a backpack.
The boys join Safety Patrol in an effort to become popular, and they try out for a contest that offers a student a chance to become the new cartoonist for the school paper. After Greg accidentally breaks Rowley's arm during a game the boys played, Rowley becomes extremely popular, and wins the contest. Greg becomes jealous of him. At Safety Patrol, Greg panicks when he encounters a truck identical to the teenagers' from Halloween, and hides the kids in a construction zone. He is spotted by a neighbour who mistakes him for Rowley. To his own bewilderment, Rowley is banned from the Patrol, but Greg eventually confesses to Rowley; Rowley accuses Greg of being selfish and ends their friendship. Rowley makes friends with Collin who replaces Greg as Rowley's new best friend.
Greg decides to pursue popularity without Rowley by joining the school play. At tryouts, Greg's soprano voice earns him the role of Dorothy Gale; however, Patty threatens the teacher into casting her instead. Greg signs up as a tree, hoping to throw apples on Patty during the play, but during rehearsal, the trees are told they won't throw apples but sing a song instead. At the performance, Greg refuses to sing as Rodrick is videotaping the performance, and begins throwing apples, ending the play in chaos. Rodrick is grounded for leaving his adult magazines out in the open where their baby brother Manny is able to find, but Greg fails to reconcile with Rowley.
One day at school, Rowley and Greg confront each other. Patty and the other kids force Greg and Rowley to fight; however, neither of them are good at fighting. The Halloween teenagers arrive at the scene and force the school to flee, but catch Rowley and Greg. They force Rowley to eat the cheese before the P.E. teacher forces them to flee. When the other kids notice that the Cheese has been moved from its original location and has been bitten, Greg covers up for Rowley by saying he ate it. This mends their friendship but makes Greg an outcast, as he is thought to have the Cheese Touch.
When the yearbooks are published, he and Rowley make the Class Favorites page as "Cutest Friends."Zachary Gordon as Greg Heffley.
Robert Capron as Rowley Jefferson, Greg's childish best friend.
Devon Bostick as Rodrick Heffley, Greg's older brother.
Steve Zahn as Frank Heffley, Greg's father.
Rachael Harris as Susan Heffley, Greg's mother.
Chloe Grace Moretz as Angie Steadman, a seventh grader at Greg's school who is a reporter for the school paper.
Karan Brar as Chirag Gupta, a friend of Greg's.
Grayson Russell as Fregley, a weird classmate of Greg's.
Laine MacNeil as Patty Farrell, Greg's arch-enemy.
Alex Ferris as Collin Lee, Rowley's substitute best friend during his fight with Greg.
Andrew McNee as Coach Malone, Greg's gym teacher.
Connor and Owen Fielding as Manny Heffley, Greg's little brother.
Belita Moreno as Mrs. Norton, Greg's acting teacher.
Filming of Diary of a Wimpy Kid was in Vancouver and wrapped up on October 16, 2009.
The official trailer for Wimpy Kid was released virally on January 21, 2010 and was shown in theaters with Tooth Fairy. A poster for the film was released shortly after. Another trailer was shown with Percy Jackson & the Olympians: The Lightning Thief.
The official Facebook account for Wimpy Kid had uploaded three clips from the film, as of March 1, 2010. In the United Kingdom and Ireland the film was released in cinemas on August 25, 2010.
The soundtrack was released on CD by La La Land Records with the score composed by Theodore Shapiro, containing 34 tracks.
A tie-in book, written by Kinney, called The Wimpy Kid Movie Diary was published on March 16, 2010, by Amulet Books (an imprint of Abrams Books). It includes film stills, storyboards, preliminary concept drawings, and also behind the scenes information to humorously chronicle the making of the film. It also includes some new illustrations.
The film was released on DVD and Blu-ray on August 3, 2010. The Blu-ray Version features six pages from Rowley's diary, Diary of an Awesome, Friendly Kid.
Review aggregator site Rotten Tomatoes, based on 101 reviews, gives the film an approval rating of 53%. The consensus given is: "Unlike its bestselling source material, Diary of a Wimpy Kid fails to place a likable protagonist at the center of its middle-school humor – and its underlying message is drowned out as a result." It also holds a "mixed or average" rating of 56 at Metacritic, based on 26 reviews. Audiences polled by CinemaScore gave the film an average grade of "A−" on an A+ to F scale. Roger Ebert gave the film three-and-a-half stars out of four, writing "It's nimble, bright and funny. It doesn't dumb down. It doesn't patronize. It knows something about human nature." Glenn Whipp of the Associated Press was less positive, saying, "In transferring the clean, precise humor of Kinney's illustrations and prose to the big-screen, the material loses just a bit of its charm." At the Movies host David Stratton gave the film one star while co-host Margaret Pomeranz gave it half a star. Stratton called the film "tiresome" and said there was "nothing remotely interesting in Thor Freudenthal's direction or the screenplay." Pomeranz disliked the character of Greg Heffley, saying "I really thought he was unpleasant. I did not want to spend time with him. I could not wait for the end of this film."
The film opened in second place at the weekend box office grossing $22.1 million, behind Alice in Wonderland.
Despite a lack of distinctive marketing, Diary of a Wimpy Kid drew a decent crowd, opening to $22.1 million on approximately 3,400 screens at 3,077 sites, notably beating out the heavily hyped The Bounty Hunter. It was the biggest start ever for a non-animated, non-fantasy children's book adaptation. Diary of a Wimpy Kid grossed more in its first three days than other film adaptions to children's novels like How to Eat Fried Worms and Hoot grossed in their entire runs. The film grossed $64,003,625 in North America and $11,696,873 in other territories for a worldwide total of $75,700,498.
Three sequels were released in 2011, 2012 and 2017 respectively. Diary of a Wimpy Kid: Rodrick Rules was released on March 25, 2011. It was based on the second book in the series, Rodrick Rules. Zachary Gordon reprised his role in the film. The film received a 47% approval rating on Rotten Tomatoes. Diary of a Wimpy Kid: Dog Days was released in August 3, 2012 and is based on The Last Straw and Dog Days, including scenes from both books. A film based on The Long Haul was released in May 2017.