Grady died at his home in Thousand Oaks after a four-year battle with cancer, said his wife, Ginny.
A native of San Diego, Grady was born Don Agrati on June 8, 1944. His father, Lou, was in the Navy and later became a sausage maker. His mother, Mary, was a talent agent.
The family moved to Lafayette in the Bay Area, where Grady developed a talent for music and dancing. He told the Contra Costa Times that he took clarinet and accordion lessons and later taught himself bass, guitar and the trumpet.
His musical talents landed him an audition with Walt Disney's "Mickey Mouse Club" when he was in middle school. He sang and danced on the show for several years but left for a part on "My Three Sons" when he was 16.
The show, which aired from 1960 to 1972, was one of the longest-running family sitcoms of all time. It featured Fred MacMurray as the thoughtful, pipe-smoking widower Steve Douglas, who raised his boys as a single parent.
"I think we did a good show," Grady said in a 2001 interview on CBS' "The Early Show." "It was a clean show. It was a fun show."
The show's wholesome portrayal of American life is what helped it resonate with families who tuned in weekly for the latest trials and tribulations in the Douglas household, according to film historian Leonard Maltin.
"America loved this family," Maltin said of the show. "It represented stability and continuity."
When the series began, Grady was 16 and played the 14-year-old Robbie. His older brother Mike was played by Tim Considine, and his younger brother Chip was played by Stanley Livingston. When Considine left the show in 1965, he was replaced by Barry Livingston, Stanley Livingston's real brother. Barry Livingston played Ernie Thompson, an orphan adopted by Steve Douglas.
"It's a cliche, but Don was the guy we looked up to because he was our big brother," Barry Livingston said Wednesday night. "The lines blur when you're working with them and living with them so many hours a day. Don was the oldest, so we were emulating him."
Grady also appeared in other television shows of the era, including the "Rifleman" and "Wagon Train."
He later began a new career as a composer and songwriter for television, theater and films.
"His passion was music," his wife, Ginny, said. "And it was because he played seven different musical instruments that he got onto the Mouseketeers show. TV was a sideline to all he ever wanted to do, which was play music."
Besides his wife of 26 years and his mother, Grady is survived by two children, Joey and Tessa, and a sister, Marilou Reichel. Another sister, actress Lani O'Grady, died of a drug overdose in 2001.
We'll have more later at latimes.com/obits.
[For the record, 8:45 p.m. June 27: An earlier version of this post incorrectly said Tom Considine played Mike on "My Three Sons." Mike was played by Tim Considine.]