The son of Dr. and Mrs. W.E. Buchanan, he was born in Humansville, Missouri, and moved with his family to Oregon when he was seven. In 1928, he earned a DDS degree from North Pacific College School of Dentistry, which later became Oregon Health & Science University School of Dentistry. His wife Mildred (1907–1987) and he, classmates in dental school, were married in 1928, the year he graduated. They had one child, a son named Buck. In 1939, they moved from Eugene, Oregon, to Altadena, California, where they moved their dental practice. He joined the Pasadena Playhouse as an actor. He appeared in his first film in 1939, at the age of 36, after which he turned his dentistry practice over to his wife. He was a member of Theta Chi Fraternity and a Freemason.
Buchanan appeared in more than 100 films, including Texas (1941) where he played a dentist and starred with William Holden and Glenn Ford and later in Penny Serenade (1941) with Cary Grant, Tombstone, the Town Too Tough to Die (1942), The Talk of the Town (1942) with Ronald Colman and Jean Arthur, The Man from Colorado (1948), Cheaper by the Dozen (1950), She Couldn't Say No (1954), Ride the High Country (1962) with Randolph Scott and Joel McCrea, McLintock! (1963) with John Wayne, Move Over, Darling (1963) with Doris Day and James Garner, and Benji (1974).
Among the many television series in which he was cast as a guest star were Cimarron City, The Californians, The Rifleman Edgar appeared in six episodes of The Rifleman, playing Grandpa Fogerty in “The Long Goodbye” (Episode 119) and Doc Burrage in the other five: “The Pet” (episode 15), “The Second Witness” (episode 23), “The Trade” (episode 24), “The Deadly Wait” (episode 26), and “The Angry Man” (episode 31)., the episode "Duel at Sundown" of Maverick with James Garner and Clint Eastwood, Leave It to Beaver (as both "Uncle Billy" and "Captain Jack"), The Twilight Zone, Riverboat (as Wingate Pardee in the 1960 episode, "Duel on the River"), Gunsmoke, Route 66, Bringing Up Buddy, Bus Stop, and The Lloyd Bridges Show.
Buchanan's roles as a regular cast member in television programs included Red Connors in the syndicated Western Hopalong Cassidy, and J.J. Jackson in the CBS crime drama Cade's County.
In 1956, Buchanan portrayed the lead in the 39-episode syndicated Western television series, Judge Roy Bean, set in Langtry, Texas, and filmed in color in California. Only loosely based on the career of the legendary storekeeper, bartender, and justice of the peace, Roy Bean, the series had supporting roles for Jack Buetel, Jackie Loughery, and Russell Hayden as a Texas Ranger. The series popularized Bean's claim to having been "The Only Law West of the Pecos", a reference to the Pecos River in Val Verde County in southwestern Texas. From 1960 to 1962, he appeared four times on the NBC Western series Laramie, Buchanan made two guest appearances on Perry Mason: in 1958 as a small-town coroner in "The Case of the Perjured Parrot" and in 1962 as a small-town judge in "The Case of the Lurid Letter".
Buchanan appeared as Uncle Joe Carson in all 222 episodes of Petticoat Junction, the only actor from the show to do so, as well as in 17 episodes of Green Acres,:416 and three episodes of The Beverly Hillbillies. Another star from Petticoat Junction and he appeared together in the 1974 movie Benji; the other "star" was Higgins the dog, which portrayed the title character.
In 1967, Dot Records released "Phantom 309" (Dot #17047), a narration by Buchanan. The 45 rpm single was backed with "Cotton Picker".
Buchanan died from a stroke complicated by pneumonia in Palm Desert, California, and was interred in the Forest Lawn - Hollywood Hills Cemetery in Los Angeles.