Sadler dropped out of high school in the tenth grade in Leadville, Colorado. He enlisted at 17 in the U.S. Air Force in 1958. He finished his GED certificate while in the Air Force. He trained as a radar technician in 1958 and was stationed in Japan. After his discharge in 1962, Sadler enlisted in the U.S. Army, and volunteered for Airborne and Special Forces, opting to be a medic.

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 Force 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 was unable to score anything close to a major success, though his "The A-Team " was a Top 30 Billboard chart single during 1966. He took his honorable discharge from the Army in May 1967, and moved with his family to Tucson, Arizona. After minor acting roles in four episodes of two TV western series--"Death Valley Days and "The High Chaparral"--and in the 1968 caper film Dayton's Devils starring Rory Calhoun, he moved to Nashville, and begin writing pulp fiction novels. His popular Casca series is about the title character, Casca Rufio Longinius (a combination of Longinus and the Wandering Jew), a legendary Roman soldier who thrust his lance into Christ's side during the crucifixion. Because of this, he is cursed to remain a soldier until the Second Coming. The novels feature Casca's life through the 20th century. Sadler wrote the first twenty-two books. After his death the series continued with books written by different authors.

On December 1, 1978, at about 11 p.m., Sadler killed a country music Songwriter named Lee Emerson Bellamy with one gunshot to the head. The shooting was the culmination of a month-long dispute the men had concerning Darlene Sharpe, who was Bellamy's former girlfriend, and Sadler's lover at the time. Bellamy made many harassing telephone calls to Sadler and Sharpe, had one violent confrontation in a Nashville bar's parking lot, and threatened both their lives.

On the night in question, Bellamy made several harassing telephone calls, including one to the Natchez Trace Restaurant where Sadler and Sharpe were having dinner and drinks with two friends. That resulted in Sadler asking a bartender to telephone the police, who never responded. Bellamy later went to Sharpe's apartment complex and knocked on the door. Sadler exited through a side door. On seeing Sadler, Bellamy fled to his van. It was at this time, Sadler testified, that he saw a flash of metal. Believing it to be a gun, he fired one shot. The bullet struck Bellamy right between the eyes, and he died several hours later in a Nashville hospital. It turned out that Bellamy was unarmed. According to court records, Sadler had then placed a handgun in Bellamy's van, presumably to strengthen his claim of self-defense. After a plea bargain, on June 1, 1979, Sadler was convicted of voluntary manslaughter for the death of Lee Emerson Bellamy, and sentenced to from 4 to 5 years in prison. His legal team worked to lower the sentence, which a judge reduced to just 30 days in the county workhouse. He served 28 days. Sadler was sued for wrongful death by Bellamy's stepson, and was ordered to pay compensation of about $10,000.

Sadler moved to Guatemala City in 1984. He continued to write and publish his Casca books, produced a never-released self-defense video, and provided free medical treatment in rural villages. On September 7, 1988, Sadler was shot in the head while sitting in a cab in Guatemala City. Witnesses said he accidentally shot himself, but his friends and family believed he was shot by a robber or an Assassin. He was flown to the United States by friends in a private jet paid for by Soldier of Fortune magazine publisher Bob Brown.

After being moved to the Cleveland VA Hospital for specialized treatment, he was removed from the hospital by two former Green Berets and his mother, Blanche Sadler. After a contentious court battle waged by his wife and children, a court in Tennessee ruled that Sadler be put under the care of an independent guardian. Sadler was moved to the VA Hospital in Murfreesboro, Tennessee, in February 1989, but he never recovered from his injury. He died there of cardiac arrest on November 5, 1989, four days after his 49th birthday. Sadler was survived by his wife, Lavona, a daughter, Brooke, and two sons, Thor and Baron.