Sadler recorded his now-famous song, "Ballad of the Green Berets," a patriotic song in ballad style. The recording of his Vietnam songs was initiated by the urging of writer Robin Moore, author of the novel The Green Berets which became a movie starring John Wayne. Moore wrote an introduction to Sadlers' autobiography I'm a Lucky One, written with Tom Mahoney (Macmillan, 1967). The title of this book is also a Sadler recorded song. The "Ballad of the Green Berets" was picked up by RCA Records in early 1966 and became a fast-selling single, reaching #1 on the yearly single charts. Sadler recorded an album of similarly themed songs, called Ballads of the Green Berets. None of the other songs on the album (which generally tell the common tales of soldiers serving in a time of war) made an impact. Sadler was widely thought to be a writer of simple songs, and having an average voice.

Unable to score another major hit, although "The A-Team" was a top 30 Billboard charted single in 1966, Sadler took to writing books. He chose to write about soldiers, but his series of novels took a turn far different from his music. His "Casca" series centers on the title character, Casca Rufio Longinius (a sort of combination of Longinus and The Wandering Jew), who stabbed Christ during the crucifixion, and is cursed to forever remain a soldier until the Second Coming. The series of 22 novels takes Casca through to the 20th century. Sadler himself wrote only the first two, with the remainder being farmed out by the publishers to other writers and issued under his name.

Later in life and after serving time in prison for a fatal shooting, Sadler moved to Guatemala City, and it was there that he was shot multiple times, several in the abdomen. He died a little more than a year later at his mother's house in Murfreesboro, Tennessee. Some believe his shooting was a result of his efforts to train and arm the Contras.

Kitabe.com has 24 audio books of the Casca Series.