His musical talents landed him an audition with "The Mickey Mouse Club" when he was in middle school. He performed on the Disney show for several years but left for a part on "My Three Sons" when he was 16.
That 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 at 837 Mill St. in a middle-class Midwestern home.
"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 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 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."
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.
After "My Three Sons" ended, Grady continued with his enthusiasm for music and began a new career as a composer and songwriter for television, theater and films.
He wrote the theme song to Phil Donahue's talk show, and his compositions were featured in the children's TV series "The Kid-A-Littles" and the 1985 film "Girls Just Want to Have Fun."
Grady also co-wrote "Keep the Dream Alive," which was recorded by Herbie Hancock, Della Reese and others for the Jazz to End Hunger project.
"His passion was music," his wife said. "TV was a sideline to all he ever wanted to do, which was play music."
Besides his wife, Grady is survived by his mother; two children, Joey and Tessa; and a sister, Marilou Reichel. Another sister, actress Lani O'Grady, died of a drug overdose in 2001.
Times staff writer Claire Noland contributed to this report.