Smith was born in Red Bank, New Jersey, the son of Grace, a homemaker, and Donald Smith, a postal worker. He has an older sister, Virginia, and an older brother, Donald Smith, Jr. He was raised in a Catholic household and attended Henry Hudson Regional High School in Highlands, The New School for Social Research in New York and the Vancouver Film School, where he met Scott Mosier, the producer of all of his films. He majored in film, but dropped out halfway through his studies, choosing to take a partial tuition reimbursement in order to help finance his first film. Smith is married to Jennifer Schwalbach Smith. He named his daughter Harley Quinn after character Harleen "Harley Quinn" Quinzel from Batman: The Animated Series. Although Smith was raised Catholic, he has said on Back To The Well, the Clerks II documentary, that now he only goes to mass on days before he starts production of a movie, and the day before it premieres. He never smoked until his debut film, Clerks, where he used the cigarettes as a prop, but never actually inhaled. In fact, he had said that prior to filming Clerks, he was a staunch non-smoker. Today, he does smoke regularly. Smith's weight has always been a defining characteristic, and has gained him unwanted media attention.
In early 2005, Smith appeared in three episodes of the Canadian-made Degrassi: The Next Generation television show. In the episodes, Smith, portraying a fictionalized version of himself, visited the school to work on the film Jay and Silent Bob Go Canadian, Eh!. Smith wrote all his dialogue for the shows he appeared in. All three episodes were collected on a DVD entitled Jay and Silent Bob Do Degrassi. In the course of filming the film, Smith fell in love with local celebrity, Caitlin Ryan, and broke-up her long-term relationship with Joey Jeremiah. Smith and Mewes also appeared in 2 more episodes the following season, when they returned to Degrassi for the Toronto premiere of Jay and Silent Bob Go Canadian, Eh!. Then, Smith appeared in the TV movie Degrassi Goes Hollywood, once again portraying a version of himself.
In addition to appearing on Degrassi: The Next Generation, Kevin Smith is an avid fan of the original Degrassi series, Degrassi Junior High and Degrassi High and references to the original are present in Clerks, Mallrats and Chasing Amy.