Mr. Grady played Robbie Douglas, the middle member of the three male siblings on the show, which ran from 1960 to 1971.
His other television credits included "The Mickey Mouse Club" (he was one of the original Mousketeers), "Wagon Train," "The Rifleman," "Have Gun Will Travel" and "Zane Grey Theater."
On the stage, his credits included a 1979 national touring production of Pippin, a Texas staging of Godspell in which he played Jesus, and regional productions of Damn Yankees, Tom Sawyer and The Boy Friend.
Mr. Grady was born Don Louis Agrati in San Francisco, CA, the son of Mary B. Castellino, a talent agent, and Lou A. Agrati, a sausage maker. He attended Stanley Junior High in Lafayette, CA, where he spent only a few months before being signed by Disney.
He is survived by his wife Ginny, his children Joey and Tessa, his sister Marilou Reichel, and his mother Mary.