Reiner was born in the Bronx, New York City, New York on March 20, 1922, the son of Irving, who was a watchmaker, and Bessie (née Mathias) Reiner. His parents were Jewish immigrants, his father from Romania and his mother from Austria. His older brother Charlie served in the 9th Division's 37th Infantry at 11 major World War II battles and had his ashes buried at Arlington National Cemetery. At age 16, Charlie read in the New York Daily News about a free dramatic workshop being put on by the Works Progress Administration and told him about it. His uncle Harry Mathias was the first entertainer in his family. He had been working as a machinist repairing sewing machines. He credits Charlie with changing his career plans.
Starting in 1960, Reiner teamed with Brooks as a comedy duo on The Steve Allen Show. Their performances on television and stage included Reiner playing the straight man in 2000 Year Old Man. Eventually, the routine expanded into a series of 5 comedy albums and a 1975 animated TV special.
In 1959, Reiner developed a television pilot titled Head of the Family, based on his own personal and professional life. However, the network did not like Reiner in the lead role for unknown reasons. In 1961, it was recast and retitled The Dick Van Dyke Show and became an iconic series, making stars of his lead actors Dick Van Dyke and Mary Tyler Moore. In addition to writing many of the episodes, Reiner occasionally appeared as temperamental show host Alan Brady. The series ran from 1961 to 1966. In 1966, Reiner co-starred in The Russians Are Coming, The Russians Are Coming.
On The Dick Van Dyke Show, he began his directing career. After the series ended its run, his first film feature was an adaptation of Joseph Stein's play Enter Laughing (1967), which, in turn, was based on Reiner's semi-autobiographical 1958 novel of the same name. Balancing directing, producing, writing, and acting, Reiner has worked on a wide range of films and television programs. Films from his early directing career included Where's Poppa? (1970), Oh, God! (1977), and The Jerk (1979).
Reiner is the author of several books, including his 2004 memoir My Anecdotal Life: A Memoir and novels, such as his 2006 novel NNNNN: A Novel. In American Film, he expressed his philosophy on writing comedy: "You have to imagine yourself as not somebody very special but somebody very ordinary. If you imagine yourself as somebody really normal and if it makes you laugh, it's going to make everybody laugh. If you think of yourself as something very special, you'll end up a pedant and a bore. If you start thinking about what's funny, you won't be funny, actually. It's like walking. How do you walk? If you start thinking about it, you'll trip."
Reiner at a ceremony for Mel Brooks on April 23, 2010
In June 2010, Reiner guest starred in Hot in Cleveland as Elka Ostrovsky's date and reprised the role in July. He made appearances on The Cleveland Show as Murray and wrote the story for the episode "Your Show of Shows", named after the program that started his career. Reiner is also the only person to have appeared on every incarnation of The Tonight Show.
On December 24, 1943, Reiner married singer Estelle Lebost. The two were married 64 years, until her death. At the time of the marriage, Reiner was 21 and she was 29. Estelle delivered the line "I'll have what she's having" in the deli scene of their son Rob's 1989 film When Harry Met Sally. She died on October 25, 2008, at age 94.
He is the father of Rob Reiner (b. 1947), poet, playwright and author Sylvia Anne (Annie) Reiner (b. 1949), and painter, actor, and director Lucas Reiner (b. 1960). Carl Reiner has six grandchildren, four from Rob and two from Lucas, and five great-grandchildren.
Reiner has described himself as a Jewish atheist. He says, "I have a very different take on who God is. Man invented God because he needed him. God is us."