Film and television
In 1998, Diaz received a contract to star in a television pilot, playing a bartender in a series titled Bronx County, after a talent scout at CBS saw him perform comedy in Seattle. The offer was a total surprise to Diaz, who initially did not believe him until he saw the talent scout possess tickets to have him fly to Los Angeles for the shoot. The series was not picked up, but the opportunity led to more work for Diaz, including an offer in his first feature film, a referee in BASEketball (1998), and a part in the television series NYPD Blue. In 2000, Diaz was featured in You Got Nothin' (2003), an independent film. Parts of Analyze That (2002), which starred Diaz, were filmed in Hudson County, New Jersey. He said, "That was big for me, (to be) able to come home to film".
Diaz's profile increased with subsequent roles in Law & Order and his first major feature films, Spider-Man 2 (2004) and Taxi (2004). After, Diaz acted as an unlawful union organizer in The Longest Yard (2005). Diaz secured the role when he learned a remake of the original was in production, and had lunch with Chris Rock and Adam Sandler, during which Rock said he could land Diaz an audition. Diaz proceeded to assemble an audition tape: "I went out and got a football jersey two sizes too small, pants two sizes too small with my butt hanging out. I got kids' football shoulder pads and ran around the field with a cigar in my mouth". Three days after submitting the tape, Diaz landed the role; the producers liked the name Big Tony Tedesco, which Diaz named himself on the tape, and wrote it into the script. Originally the part had a mere three lines, but Diaz's tape got his part extended.
In 2005, Diaz expressed his wish to continue as a character actor, saying: "It's like a dream come true for me. ... I got my call and I have to make the most of it". Around that time, Diaz prepared his one-man comedy show Larceny & Laughter, which featured stories about his time growing up in North Bergen.
In 2007, Diaz appeared in four episodes of the television series My Name is Earl as Joey the Candy Bar Criminal. The show's producers wished to hire actors to play prisoners, and called Diaz after they saw him in The Longest Yard. Also in 2007, he hosted a series of humorous promotions for the Ultimate Fighting Championship as Joey Karate, giving karate instructions and comical predictions for upcoming fights. He regards his role in the 2009 television film The Dog Who Saved Christmas as a point of pride, as it allowed children in North Bergen to see him act. At the time of filming, Diaz weighed 390 lbs. By late 2009, he slimmed down to 295 lbs. In 2011, Diaz appeared in "Scarlet Ribbons", an episode of The Mentalist, and the film Bucky Larson: Born to Be a Star.
Around 2011, Diaz raised $1,400 from fans on Twitter and Facebook to finance a documentary about his upbringing in North Bergen, chronicling his life and how he was influenced by the various people who took care of him after his parents' death. Diaz contributed $2,600 to complete it and by mid-2012, Where I Got My Balls From was produced with his podcast co-host and producer Lee Syatt as director, who shot six hours of footage in total. Diaz made the film in tribute to those who helped him.
In 2013, Diaz appeared in "Sal's Pizza", the ninth episode of the comedy Brooklyn Nine-Nine. In the same year, he played Robert De Niro's coach in the feature film Grudge Match, which premiered on December 25. Diaz took his seventh-grade teacher to the premiere. In 2014, Diaz appeared in "White Truck", the twelfth episode of the second season of Maron. In 2017, he started work on a presentation based on one of his cats for Animal Planet.