Into his teenage years, Quintel loved drawing and watching cartoons such as The Simpsons, Beavis and Butt-Head and The Moxy Show, as well as British shows such as The League of Gentlemen and The Mighty Boosh. He often played the video game ToeJam & Earl, the influence of which Quintel later described as "the perfect platform for Mordecai and Rigby" characters of Regular Show. Quintel also became influenced by rock music from the 1980s and later added 1980s music into Regular Show.

In 1997, Quintel began attending Hanford High School. In 1998, when Quintel was 16 and a sophomore at Hanford High, Quintel's father James gave Quintel a video camera, which Quintel used, along with Lego men and crude paper cutouts, to create a few minutes of stop motion film for several short film projects at Hanford High School. To expand his artistry, Quintel took an AP literature class and a pottery class at Hanford High as well as took a summer class where Quintel learned how to animate films and make flip books. Quintel also worked at a movie theater and at "a lot of minimum wage jobs," just as 23-year-olds Mordecai and Rigby work for minimum wage at a park. In May 2000, Quintel was nominated as a 12th grade California academic all-star from Hanford High School.

During the fall of Quintel's fourth year at CalArts, The Naive Man from Lolliland won both the Producers Choice Award (an Apple G5 computer and a copy of Bauhaus Software's Mirage animation software) and the Student Animator Award (US$1,000 and a copy of Softimage XSI 3D computer graphics software) at the 2005 Nextoons Nicktoons Film Festival. In response to Quintel's success at the 2005 Nicktoons Film Festival, Fred Seibert, an entertainment Entrepreneur and television and film Producer, identified Quintel as "an original talent to watch out for in the future". Quintel's hometown local newspaper, the Hanford Sentinel, noted Quintel's success at the 2005 Nicktoons Film Festival as being one of 2005's moments from a memorable year. In December 2005, Quintel graduated from the California Institute of the Arts with a BFA degree in character animation.

At about this same time, Quintel passed a test and began working as a storyboard revisionist for Cartoon Network's Camp Lazlo. In May 2006, Nicktoons announced that Quintel would be one of six judges at the 3rd Annual Nicktoons Network Animation Festival. In 2007, Quintel entered his short film, 2 in the AM PM, in the 30th annual Spike & Mike's Sick and Twisted Festival of Animation. In 2 in the AM PM, two slackers are left alone to run a convenience store/gas station on Halloween night, where candy filled with drugs creates a mini-nightmare. Quintel later used some of these 2 in the AM PM characters in Regular Show.

In December 2009, ASIFA-Hollywood nominated Quintel for an Annie Award in the category of "Directing in a Television Production" for his directing work on an episode of The Marvelous Misadventures of Flapjack. In September 2011, the Academy of Television Arts & Sciences nominated Quintel for a Primetime Emmy Award in the Outstanding Short-format Animated Program category for Regular Show. He most recently worked for Cartoon Network Studios in Burbank, California developing episodes for Regular Show.

In 2008, Quintel pitched Regular Show for the Cartoonstitute project at Cartoon Network by using a storyboard, with his reasoning that "I don't think me verbally pitching such an odd concept would have made any sense to anyone." In 2009, Cartoon Network ordered more episodes of The Marvelous Misadventures of Flapjack as well as greenlit Quintel's project, Regular Show. The agreement upon the premise of Regular Show was that the series would be about two park groundskeepers, Mordecai (a 6-foot blue jay) and Rigby (a hyperactive raccoon), who try to entertain themselves at their jobs while doing anything they can to avoid work and escape their everyday boredom. Along with his success at getting approval to develop Regular Show, in December 2009, Quintel and Director John Infantino were nominated for an ASIFA-Hollywood Annie Award in the category of "Directing in a Television Production" for their directing work on the Candy Casanova episode in Season 2 of The Marvelous Misadventures of Flapjack. However, American animation Director and fellow CalArts graduate Bret Haaland subsequently took the Director Annie Award in February 2010 for Haaland's work on The Penguins of Madagascar – Launchtime. Nine months after Quintel was nominated for an Annie Award, Regular Show debuted in New York at 8:15 P.M. EST on September 6, 2010. In September 2011, while the first season of Regular Show was being aired, Quintel was nominated for a Primetime Emmy Award in the Outstanding Short-format Animated Program category as executive Producer and creator of Regular Show. However, his Regular Show series was beat out for the award by the ABC animated special, Disney Prep & Landing: Operation Secret Santa.

By March 2012, Quintel was directing a crew of about 35 to develop each episode of Regular Show, which takes about nine months to go from idea to final product. Quintel has most recently worked for Cartoon Network Studios in Burbank, California developing episodes for Regular Show and provided his real voice to the 23-year-old blue jay Regular Show character Mordecai. In commenting on his voice acting, Quintel noted how he has an easy-going nature and never yells in real life, so he had to learn how to yell as the Mordecai character.

In 2017, it was announced that TBS has picked up a new animated series by Quintel, entitled Close Enough, a co-production of Cartoon Network Studios with the newly-established Studio T.