He was 89.
Bain's daughter told Access Hollywood he died Monday of natural causes in his hometown of Livermore, California.
"He was an amazing person," Jennifer Bain told TMZ. "He was a lot like Mr. Drummond, but much more interesting in real life. He was an amazing father."
There will be no funeral or public memorial.
Bain had already played a major role in the 1970s sitcom "Maude," but he became a household name starring as Phillip Drummond, the white millionaire who adopts the two black sons of his deceased housekeeper, according to the "TODAY" show.
However, all three of the child actors later fell on hard times, and now just one, Bridges, is still alive.
Plato died of a drug overdose in 1999 at age 34, and Coleman, who suffered from kidney problems, died in 2010 amid legal and financial troubles. Bridges has also had drug problems and a lengthy arrest record.