James began doing stand-up comedy in 1989, making his debut at the East Side Comedy Club on Long Island, managed by Richie Minervini. He gained popularity through numerous appearances on various talk shows, including The Tonight Show with Jay Leno, Late Show with David Letterman, Late Night with Conan O'Brien, Dennis Miller Live, The Late Late Show, The Rosie O'Donnell Show, The Ellen DeGeneres Show, and Live with Regis and Kathie Lee. James was listed at #89 on Comedy Central's list of the 100 Greatest Stand-Up Comedians. James has also done his stand-up routine on Just for Laughs, an annual comedy festival in Montreal, Quebec. Later he was on commercials for Mazzio's Italian Eatery. In 2001, James did his own stand-up special called Kevin James: Sweat the Small Stuff. He has also appeared as a musical guest on Just for Laughs.

James' first television job was in 1991 on The New Candid Camera, where he used his comedy timing and improvisation skills playing the actor that pulled the practical jokes on unsuspecting people. James appeared on television as the announcer for the MTV Sports game show SandBlast from 1994-96. James later moved to Los Angeles and befriended Ray Romano, and later guest-starred on a few episodes of Romano's hit CBS sitcom, Everybody Loves Raymond. These appearances led to the development of his own sitcom, The King of Queens, which ran on the same network from September 21, 1998, to May 14, 2007. James played working class parcel delivery man Doug Heffernan who works for a company known as IPS. He is married to Carrie (Leah Remini), a sharp-tongued, ambitious secretary at a Manhattan law firm who is far less content with working-class life in Queens. Her obsessive, vindictive father, Arthur (Jerry Stiller), who is capable of bizarre conduct, lives with them. For his work on the eighth season, James was nominated for a Primetime Emmy Award for Outstanding Lead Actor in a Comedy Series in 2006.

On June 19, 2004, James married Actress Steffiana de la Cruz in California. They have four children.

James made his film debut in the 2005 romantic comedy Hitch, alongside Will Smith and Eva Mendes. In 2006, he co-starred with his Everybody Loves Raymond colleague Ray Romano in the comedy Grilled, and provided voice work in the animated films Monster House and Barnyard. James co-starred with Adam Sandler in the comedies I Now Pronounce You Chuck and Larry (2007), You Don't Mess with the Zohan (2008), and Paul Blart: Mall Cop (2009). In the latter film, his character patrolled the fictional West Orange Pavilion Mall, filmed at the Burlington Mall in Burlington, Massachusetts, on a Segway PT. The film opened as the #1 film in North America with a weekend gross of $39 million, despite overwhelmingly negative reviews, and eventually grossed $219 million from ticket and home video sales. More recently, James appeared in Grown Ups, which co-starred numerous Saturday Night Live alumni and was even more universally panned, yet was highly successful at the box office.

In 2007, he was the grand marshal for the Pepsi 400, saying "Gentlemen, start your engines". He did it again in June 2010 with friend and fellow actor Adam Sandler to promote Grown Ups, in 2012 at the fall Talladega race to promote Here Comes the Boom and in July 2013 at the Coke-Zero 400 with co-stars Adam Sandler and Shaquille O'Neal.

A web series titled Dusty Peacock produced by James and starring his brother Gary Valentine began streaming on Crackle in July 2009.

James hosted the 2010 Nickelodeon Kids' Choice Awards on March 27, 2010, and was a nominee into the Arm Fart Hall of Fames of the next year's show as Kevin "Not-Quite-As-Good-As-Me" James; but lost to Josh Duhamel.

James is the younger brother of Comedian Gary Valentine (born Gary Knipfing), who played Doug's cousin Danny Heffernan on King of Queens. Kevin and his siblings were raised in the Catholic faith, which, Kevin stated in 2012, he continues to practice.

In 2011 he had a lead role with Vince Vaughn in the comedy-drama The Dilemma and then with Rosario Dawson in Zookeeper, which he wrote and produced. James also had the lead role in the 2012 mixed martial arts comedy film Here Comes the Boom. He starred as well in the movie Pixels in 2015 and the following year in True Memoirs of an International Assassin, directed by Jeff Wadlow and released by Netflix on November 11, 2016. James in 2017 co-starred with Adam Sandler in another Netflix film, Sandy Wexler.