Cameron Diaz's father was on set during filming, so the Farrellys decided to throw him in the movie. In the scene where Ben Stiller is released from prison after being wrongfully accused of murder, Cameron's dad is the inmate in the front with a beard and long hair jeering at Ben.
Lee Evans' British accent in the movie was characterized by a film critic as quite possibly the worst fake British accent in a movie he had ever heard. Lee Evans' is in fact British, however his normal accent is the Bristol accent of British as opposed to the snobbish posh version he used in the movie. While this means his accent is in all actuality not fake, it was exaggerated for the role.
After the financial losses suffered from Kingpin (1996), the Farrely brothers thought their next film would probably be their last. So they decided to go all out and deliver the most hysterically black comedy they could dream up. When There's Something About Mary (1998) became a box-office smash hit, the Farrely's careers were safe to continue.
The dialogue between Mary and Ted about how there aren't enough meats on sticks was originally written for an episode of Seinfeld (1989) that never aired. The Farrelly Brothers liked it and bought it for use in the movie.
Ted brings Warren a baseball autographed by Tony Conigliaro. Conigliaro played for the Boston Red Sox in the 60s and was seriously injured when he was hit in the face by a pitched fastball, echoing Warren's accident in the previous montage.
The studio was initially reluctant to allow Ben Stiller - the Farrelly Brothers' first choice - to star, so the brothers decided upon a then unknown Owen Wilson instead. When the studio was even more reluctant to let Wilson star, they agreed to allow the Farrellys to cast Stiller.
The waterfront house in which Mary lives in Miami was destroyed in 2008 by a construction accident that killed two people and injured five others. A crane collapsed at a nearby 40-story condominium under construction going through the roof of the house. The developer had originally designed to incorporate the house to be part of the property of the condominium development. As a result of the accident and the 2008 housing recession, the condominium never opened.
The scene with the homosexuals is staged in a South Carolina Rest Stop (with South Carolina trooper decals and uniforms). This is based on a real case where Troopers discovered that the stops where being used as meeting places for homosexuals to meet and have anonymous sex. The troopers learned this after arresting a South Carolina state offical "in the act" at one of the stops.
The high school scene in the beginning was shot at Plantation's City Hall in Florida. Upon seeing a rough cut of the film, and displeased with the raunchiness, city council members requested that Plantation not be acknowledged in the credits.
The character "Patrick Healy" is also the name of an associate producer involved with this film. It is also the name of the National District Attorneys' Association spokesman in the book, "Fear and Loathing in Las Vegas" by Dr. Hunter S. Thompson.
One detective in the scene at the police station with Ben Stiller says, "I'm Detective Stabler, this is Detective Krevoy." These names are a reference to the film's producers, Steven Stabler and Brad Krevoy.
Although the identity of Brett Favre as Mary's previous boyfriend "Brett" is kept secret throughout the film, a clue to his identity is provided by Mary's friend who refers to him as "Pacman". At the time, Favre played for the Green Bay Packers.