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Bain, who starred opposite the young Gary Coleman on the NBC sitcom as his adoptive father, Philip Drummond, died of natural causes at a comfort-care facility in Livermore, California, east of San Francisco, on Monday (local time). He was three weeks shy of his 90th birthday, according to his daughter, Jennifer.
Born in Alberta, Bain served in the Canadian Army during World War Two, became a U.S. citizen in 1946 and went on to a career as an actor on Broadway and television. He often played erudite, professional characters such as lawyers, executives, politicians or doctors.
Following a recurring role on the daytime vampire drama "Dark Shadows" as an innkeeper, Bain broke into prime-time comedy with a supporting role on Norman Lear's "All in the Family" spin-off "Maude," which starred Bea Arthur in the title role.
On "Maude," Bain played a conservative physician and next-door neighbour, Dr. Arthur Harmon, who was frequently at political odds with the outspokenly liberal Maude but was best friends with Maude's husband, Walter.
At the end of that show's six-year CBS run in 1978, Bain landed his own sitcom, "Diff'rent Strokes," in which he played Drummond, a rich, widowed industrialist who takes in the two young sons of his housekeeper after she dies, creating a racially mixed family in an era when depictions of such households were rare on TV.
Joining Drummond's 13-year-old daughter, Kimberly, and a ditzy new housekeeper, Mrs. Garrett, the two boys, precocious 8-year-old Arnold, played by Coleman, and his quieter 12-year-old brother, Willis, find themselves in the lap of luxury as they adjust to a new life on Park Avenue.
The show ran for eight seasons, 1978-1986, on NBC, and went into wide re-run syndication around the world. Coleman's oft-repeated line to his brother, "What you talkin' 'bout, Willis?" became a pop culture catchphrase.
Coleman, who grappled with a series of financial, legal and domestic woes later in life, died in May 2010 at age 42 after suffering a brain haemorrhage.
Bain returned periodically to the stage during the show's network run and reprised the Philip Drummond role on a 1996 episode of "The Fresh Prince of Bel-Air," which starred Will Smith as a young rapper from a tough Philadelphia neighbourhood who ends up living with wealthy relatives in California.
Bain also briefly co-starred on prime-time TV in the 1987-88 season in the Fox network political comedy "Mr. President," as the loyal chief of staff to the title character, played by George C. Scott.
Bain is survived by his daughter and two sons, Mark and Kent.