When he completed Airborne training, he underwent the lengthy training as a combat medic at Fort Sam Houston, Texas, Fort Jackson, South Carolina, and Fort Bragg, North Carolina. In May 1965, while he was on a combat patrol southeast of Pleiku in the Central Highlands, he was severely wounded in the knee by a feces-covered punji stick. Sadler dressed the wound with a cotton swab and an adhesive bandage, then completed the patrol; however, he subsequently developed a serious infection in his leg, and was evacuated to Clark Air Base Hospital in The Philippines. Doctors were forced to surgically enlarge the wound to drain it and to administer large doses of penicillin. Sadler returned to Fort Bragg where he made a complete recovery.
Sadler recorded his famous song, "The Ballad of the Green Berets", a patriotic tune about the Special Forces, in December 1965. The writer Robin Moore, author of the novel The Green Berets, which became a 1968 movie, The Green Berets, featuring John Wayne, helped Sadler with lyrics and with getting a recording contract with RCA Victor Records. "The Ballad of the Green Berets" arranged as a choral version by Ken Darby was the title song of the movie. Moore also wrote an introduction to Sadler's autobiography, I'm A Lucky One, which he dictated to Tom Mahoney and was published by Macmillan Company in 1967. "The Ballad of the Green Berets" was released by RCA Victor Records in January 1966, and became a fast-selling single, holding No. 1 on the Billboard Hot 100 chart for five consecutive weeks from March 5 to April 2, 1966. The record sold more than nine million copies. The song was a great success in many U.S. cities; it scored five weeks at No. 1 on the weekly Good Guys music survey at WMCA, the top popular music radio station in New York during 1966. He sang it for his television début on January 30 on The Ed Sullivan Show. Sadler had recorded an album of similarly themed songs which he titled Ballads of the Green Berets. It sold a million copies during the first five weeks of its release. Sadler had a minor success, the similarly patriotic-themed "The 'A' Team", later the same year when that single scored #28.