Kevin Patrick Smith was born on August 2, 1970 in Red Bank, New Jersey, the son of Grace (nee Schultz), a homemaker, and Donald E. Smith, a postal worker. He has an older sister, Virginia, and an older brother, Donald Smith, Jr. He was raised in a Catholic household, in the nearby clamming town of Highlands. As a child, Smith's days were scheduled around Donald's late shifts at the post office.

After the success of his first films, Smith lived in Los Angeles, though he disliked no longer being near his New Jersey friends. He dated actress Joey Lauren Adams, and declared his desire to marry her in Time magazine, but they began to grow apart after he spoke of staying in Los Angeles permanently and starting a life with her. When Smith's grandmother became ill, he returned to Red Bank and remained there, realizing that that was where he belonged. Smith and Adams' relationship was tested by their working together on Chasing Amy, the set of which saw a heated argument between the two. They broke up in June 1997.  Smith is married to Jennifer Schwalbach Smith, whom he met while she was interviewing him for USA Today. They got married at Skywalker Ranch on April 25, 1999. He photographed her for a nude pictorial in Playboy that consisted of photographs by various celebrities. Their daughter, Harley Quinn Smith, was born June 26, 1999, and was named after the character from the Batman comics. They live in the Hollywood Hills, in a house Smith purchased from longtime friend Ben Affleck in 2003.  Initially raised Catholic, Smith began to develop doubts about his beliefs in his early 20s, and came to see Catholic Mass as "dry and lip-servicey." Seeking out advice, Smith spoke to a priest, who analogized faith to liquid filling a shot glass, and explained that the glass grows in size as a person grows older, and thus the same knowledge that satisfies a person as a child can be insufficient as an adult. Smith read extensively on Christianity, explored other religions, read the Biblical apocrypha, and tried joining a Pentecostal congregation. The thoughts and ideas he explored during this time formed the inspiration for his film Dogma, the beginning of which features characters using the shot glass metaphor used by the priest. Though Smith still regularly attended Mass as late as 1998, he stated on "Back to the Well", a feature on the Clerks II documentary, that now he only goes to Mass on the day before he starts production of a film, and the day before it premieres. Nevertheless, Smith told the BBC in September 2014 that he firmly professes a belief in God. He said, "Proof of God is that I have a career."  Smith is an avid hockey fan and loyal New Jersey Devils fan. He is also a fan of the Edmonton Oilers.

Do they have any children?
Their daughter, Harley Quinn Smith,