Carl William Demarest was born in Saint Paul, Minnesota, the son of Samuel and Wilhelmina (née Lindgren) Demarest. They moved to New Bridge, a hamlet in Bergen County, New Jersey, during his infancy.
Demarest served in the United States Army during World War I.
Demarest started in show business working in vaudeville, appearing with his wife Estelle Collette (real name Esther Zychlin) as "Demarest and Colette", then moved on to Broadway. Demarest worked regularly with director Preston Sturges, becoming part of a "stock" troupe of actors that Sturges repeatedly cast in his films. He appeared in ten films written by Sturges, eight of which were under his direction, including The Lady Eve, Sullivan's Travels and The Miracle of Morgan's Creek.
Demarest was such a familiar figure at the Paramount studio that just his name was used in the movie Sunset Boulevard as a potential star for William Holden's unsold baseball screenplay.
Demarest appeared with veteran western film star Roscoe Ates in the 1958 episode "And the Desert Shall Blossom" of CBS's Alfred Hitchcock Presents. In the story line, Ates and Demarest appear as old timers living in the Nevada desert. The local sheriff, played by Ben Johnson, appears with an eviction notice, but he agrees to let the pair stay on their property if they can make a dead rosebush bloom within the next month.
In 1959, Demarest was named the lead actor of the 18-week sitcom Love and Marriage on NBC in the 1959–1960 season. Demarest played William Harris, the owner of a failing music company who refuses to handle popular rock and roll music, which presumably might save the firm from bankruptcy. Joining Demarest on the series were Jeanne Bal, Murray Hamilton and Stubby Kaye.
Demarest appeared as Police Chief Aloysius of the Santa Rosita Police Department in the film It's a Mad, Mad, Mad, Mad World (1963), as well as on a memorable episode ("What's in the Box") of Rod Serling's The Twilight Zone as a hen-pecked husband driven to the murder of his wife.
His most famous television role was in the ABC and then CBS sitcom My Three Sons from 1965 to 1972, playing Uncle Charley O'Casey. He replaced William Frawley, whose failing health had made procuring insurance impossible. Demarest had worked with Fred MacMurray previously in the films Hands Across the Table (1935), Pardon My Past (1945), On Our Merry Way (1948), and The Far Horizons (1955) and was a personal friend of MacMurray. Also, he worked with Irene Dunne in Never a Dull Moment.
Demarest received a single Academy Award nomination, for his supporting role in The Jolson Story (1946), playing Al Jolson's fictional mentor. He had previously shared the screen with the real Al Jolson in The Jazz Singer.
Demarest also received an Emmy nomination for the 1968–1969 season of My Three Sons as Best Supporting Actor in a Comedy Role.
Demarest has a star on the Hollywood Walk of Fame for his contributions to motion pictures, bestowed upon him on August 8, 1979 by the Hollywood Chamber of Commerce. In attendance at the ceremony and then later at Musso & Frank Grill for celebrations were his My Three Sons co-stars Fred MacMurray and wife June Haver, Tina Cole, Stanley Livingston, Barry Livingston, and Dawn Lyn.
In 1998, a Golden Palm Star on the Palm Springs, California, Walk of Stars was dedicated to him.
Demarest was married twice. His first wife was his vaudeville partner Estelle Collette (October 26, 1886–November 19, 1968), born Esther Zichlin. Demarest helped raise her daughter from her earlier marriage to poet and novelist Samuel Gordon (September 10, 1871 – January 10, 1927), author Phyllis Gordon Demarest (b. March 31, 1908). His second wife was Lucile Thayer (September 30, 1912–October 16, 2009), born Lucile Theurer, daughter of Herman Theurer and Lillie Sjoberg, who due to her activism on health issues in the motion picture industry in October 1960 was named California Lay-Chairman of the ANA fundraising campaign.
Demarest's favorite recreations were hunting, fishing, golf, and playing the cello.
He died in Palm Springs, California and was interred in Glendale's Forest Lawn Memorial Park. At the time of his death, he was suffering from prostate cancer and pneumonia.A Night at Coffee Dan's (1927) as M.C.
Amateur Night (1927) as Theatre Manager
The Night Court (1927) as Defense Counsel (uncredited)
Seeing Things (1930)
The Run Around (1932)
The Danny Thomas Show in 5 episodes (1957–1958) as Mr. Daly
The Rebel in "The Hope Chest (1960) as Ulysses Bowman
Love and Marriage (1959–1960) as William Harris
Going My Way in "The Slasher" (1963) as Marty
Bonanza in the episode "The Hayburner" (1963) as Enos Milford
Bonanza in the episode "Old Sheba" (1964) as Angus Tweedy
The Twilight Zone in the episode "What's in the Box?" (1964) as Joe Britt
My Three Sons (215 episodes, 1965–1972) as Uncle Charley O'Casey
McMillan and Wife [Two Dollars on Trouble to Win] S2/Ep07 (1973) as Uncle Cyrus