Joseph Edward Duncan III was born on February 25, 1963, in Fort Liberty, North Carolina. He was the fourth of five children in his family. His father, Joseph Edward Duncan Jr., served in the United States Army, which meant that the family moved frequently. They changed locations every year or two, relocating both within the U.S. and abroad. When Duncan was about 12 years old, the family settled in Tacoma, Washington, after his father retired.

Duncan's mother, Lillian Mae Duncan, was often described as strict and domineering. In 1979, Duncan's parents separated, and they divorced in 1983. After the separation, Duncan's three older sisters left home all at once. He remained with his mother, while his younger brother eventually moved in with their father. Duncan's father later remarried and formed a new family.

During his school years, Duncan attended Lakes High School but did not graduate. By the age of 15, he was already involved in criminal activities. He claimed he had suffered childhood abuse; however, his brother disputed this. After his early trouble with the law, including a car theft in 1979, Duncan was sent to a juvenile rehabilitation facility.

Duncan's family life was marked by instability due to his father's military career and his mother's strict nature. This environment may have contributed to his later behavior and the violent path he would take in life.

On May 16, 2005, authorities found three bodies in a house along Lake Coeur d'Alene, Idaho. The victims were Brenda Groene, her boyfriend Mark McKenzie, and her son Slade Groene. Two other children, Dylan and Shasta, were missing. An AMBER Alert was issued. After a search, Shasta was seen at a Denny's restaurant on July 2, 2005. A man with her was later identified as Joseph Edward Duncan III. He was arrested without incident. When found, Shasta was reunited with her father, but Dylan remained missing.

On July 4, 2005, human remains were discovered in the Lolo National Forest in Montana. The remains were later identified as Dylan's. During the trial, it was revealed that Duncan killed him with a shotgun. Much of the information about the Groene family murders came from Shasta’s statements to investigators. She said Duncan killed her family after breaking into their home. Duncan bound Brenda and her family with zip ties and placed Shasta and Dylan outside. After hearing sounds from inside, Shasta saw her brother injured and then overheard Duncan bludgeoning her family members.

After abducting the children, Duncan took them to various campsites where he repeatedly abused them for six weeks. Shasta reported that Duncan eventually shot Dylan. He claimed it was an accident, saying he was looking for beer when the gun went off. Afterward, he threatened to kill Shasta but stopped when she begged him to. Eventually, they drove to the Denny's where Shasta was rescued.

Duncan had a history of violence and sexual crimes. He was also linked to the murders of Anthony Martinez in California and the deaths of two young girls, Sammiejo White and Carmen Cubias, in Washington. After his arrest, he confessed to several of these crimes.

Duncan faced multiple trials, first in Idaho, where he pleaded guilty to murders and kidnappings. In federal court, he was found guilty of federal kidnapping and related charges, receiving a death sentence. He was also extradited to California, where he pleaded guilty to the murder of Anthony Martinez.

In prison, Duncan continued to draw attention to his case and died from cancer on March 28, 2021, while on death row. His crimes affected many lives and left a lasting impact on the communities involved.