C.K. was born in New York City, with a father descended from Jewish Hungarians and Catholic Mexicans, and a mother with Irish Catholic ancestors. After four years in Mexico, he grew up in Massachusetts, in Framingham and, mostly, Newton. His family pronounce the surname "Szekely" roughly as SAY-kay (see Székely), and in his grade-school years he resorted to "C K" as an easy way to get his name pronounced almost correctly as "SEE-kay.

His credits as a writer include Saturday Night Live, Late Show with David Letterman, Late Night with Conan O'Brien, and the Chris Rock Show. He wrote and directed the movie, Pootie Tang, based on a sketch from the latter. His work for the Chris Rock Show was nominated for an Emmy Award 3 times, including winning "Best Writing in a Variety or Comedy Series" in 1999.

As a voice actor he portrayed Brendon's estranged father, Andrew Small, in Home Movies, and appeared numerous times on Dr. Katz, Professional Therapist.

He has performed his stand-up frequently on shows like Late Night with Conan O'Brien and the Tonight Show with Jay Leno. Most recently his subject matter reflected his frustrations and anxieties from raising his two-year-old daughter.

In the spring of 2005, C.K. created, with executive producer Mike Royce, a pilot for a sitcom to be entitled Lucky Louie on HBO. In September -- after doing an August half-hour HBO stand-up special as part of the stand-up series One Night Stand -- he began production on twelve episodes, as the rest of first season of 13 airing starting in June, 2006. The show is filmed in front of a live studio audience; it will be a HBO's first series in that format, and the movie lot being used for taping the first season is the same one where Desi Arnaz made the multi-camera live-audience approach a hallmark of the I Love Lucy show. Lucky Louie is described as a realistic portrayal of family life; some summarize it as "Everybody Loves Raymond with swearing".