Grady was born Don Louis Agrati in San Diego, California, the son of Mary B., a talent agent, and Lou Anthony Agrati, a sausage maker. His sister Lanita became an actress known professionally as Lani O'Grady. He grew up in Lafayette, California, before being signed by Walt Disney and leaving the area. He graduated from Burbank High School in 1962. His acting credits included several Western series, including John Payne's The Restless Gun, Have Gun – Will Travel, Buckskin, Wagon Train and The Rifleman, in which he played two roles in the second season. He played Jeff, son of a barber that got into fights with Mark McCain. This episode was called The Patsy and originally aired September 29, 1959. In December 1959, at the age of 15, Grady appeared in two different episodes of CBS's Dick Powell's Zane Grey Theatre, having played opposite Joan Crawford and Dick Powell. In "Rebel Ranger", he was cast as Rob Faring, the young son of Crawford's character, Stella Faring, a Confederate widow who tries to reclaim her former home and Rob's birthplace from the Unionist owner, Cass Taggart, played by Scott Forbes. Character actorJohn Anderson is cast as Fisk Madden, who tries to drive Taggart off his land and gain Stella's favor. The episode ends with Stella and Rob heading into a nearby town with the understanding that Taggart would call upon Stella for possible courtship, even though Crawford was fifteen years Forbes' senior. Stella Faring reveals that she had been living in San Antonio, Texas, during the war, also the birthplace of Joan Crawford. In "Death in a Wood", Grady played a young Unionist, Zachary, who grows to understand that a Confederate soldier, Lawrence, who is holding him prisoner, is a man of decency and strength of commitment. Simon Oakland appeared in this episode as a less sympathetic Confederate named Townsend. In 1960, he began an 11-year run as Robbie Douglas in the show My Three Sons starring Fred MacMurray. Initially airing on ABC, the show moved to CBS in 1965. During production of My Three Sons, Grady both appeared with his own band The Greefs on the series, writing two original songs for the show and was the drummer for The Yellow Balloon, whose self-titled song became a minor hit in 1967. For a while during the run of the series, he attended Los Angeles City College. While on My Three Sons Grady also appeared in the NBCmedical drama, The Eleventh Hour and in two episodes of the NBC education drama, Mr. Novak, starring James Franciscus. Ultimately, the frustrated middle brother on My Three Sons, Grady's role of Robbie became the confident eldest brother with two cast changes. The character of the original oldest brother, Mike was gradually eased out of the series, and a new youngest son, Ernie, was adopted into the family.
Grady's first marriage to Julie Boonisar ended in divorce in 1979. The couple had no children. Grady married Virginia "Ginny" Lewsader in 1985, having met her at Disneyland. The couple had two children, Joey and Tessa; and they remained married for 27 years, until Grady's death on June 27, 2012. He died of myeloma at age 68 in Thousand Oaks, California, and his gravesite is at the Pierce Brothers Valley Oaks Memorial Park in Westlake Village, California.
