John McGreevey began writing plays and radio scripts while a student in English/Theatre at Indiana University, 1938‑1942. Moving to Arizona shortly after his marriage in 1944, McGreevey worked as a writer and announcer for radio station KTAR in Phoenix, as well as creator, writer, and director of Arizona Adventure, a southwestern regional network radio show from 1948‑1954. He also had several short stories published in magazines and produced radio scripts for such network shows as Armstrong's Theatre of Today, Calvalcade of America, Dr. Christian, and Suspense.

During the early 1950's McGreevey began concentrating on live television, writing dramas for Armstrong Circle Theatre, Lux Video Theatre, Westinghouse Studio One, and Climax! He produced more scripts for the latter series than any other writer. In 1955 he made the transition to film television and moved to Southern California. Since then he has written several hundred scripts for a wide variety of television series, as well as movie scripts and adaptations for the stage. His most prominent television credits include co–creator and head writer for Black Saddle, 1958–1961; co–creator and Emmy nominee for The Farmer's Daughter, 1961–1964; over 50 scripts for My Three Sons, 1960–1967; and more than 30 stories for Family Affair, 1966–1970. Other shows to his credit include Adventures in Paradise, Bat Masterson, The Flying Nun, General Electric Theater, Gidget, Grindl, Hazel, Ironside, Laredo, Mayberry R.F.D., Movie of the week, The Name of the game, Owen Marshall, To Rome, with Love, Trackdown, Virginia City Editor, Wagon Train, and Zane Grey Theatre.

Probably the best known of McGreevey's writing credits was the series The Waltons, for which he wrote more than 20 of the episodes aired and won an Emmy in 1974 for a special two hour show the previous year, "An Easter Story." Concurrent with The Waltons was a growing trend of docu–drama television programming. McGreevey wrote several of these dramas, the most outstanding of which include Judge Horton and the Scotsboro Boys (broadcast in 1976; winner of four Christopher Awards, an Emmy nomination for McGreevey, and the George Foster Peabody Award); The Disappearance of Aimee; Ritchie; and the four–hour mini–series, Murder in Texas. McGreevey also wrote the screenplay for the full–length feature film Night Crossing released by Disney Productions in 1981.