Spangler grew up in Ames, Iowa. His father was a civil engineer, and there's a lab at Iowa State University named after him.

In high school, Spangler played football and met his first wife, Nancy. They got married in 1955 and moved to Littleton, Colorado. They had two kids: a son named David in 1961 and a daughter named Susan in 1963.

By 1978, Spangler was working for the American Water Works company. Throughout his career, he also worked for Honeywell's camera and instruments division, was a public relations director for a non-profit, and worked part-time as a disc jockey at a radio station. He even acted in Theatre Arts.

On the morning of December 30, 1978, in Littleton, Colorado, the bodies of 45-year-old Nancy Spangler, her 17-year-old son, and 15-year-old daughter were found in their home. All three had been shot with a .38-caliber handgun. The daughter, partially clothed, was shot in the back; the son was shot in the chest; and their mother was found with a bullet wound on her forehead, slumped over a typewriter with a typewritten suicide note signed with her initial.

Robert Spangler, age 45, admitted to having marital problems, an affair, and plans to leave his wife. He claimed he was not home when the bodies were discovered, but his story changed during interviews. Polygraph tests were inconclusive, but gunshot residue was found on his right palm. The gun used was Spangler's. On January 3, 1979, the coroner ruled the case a murder-suicide, and the sheriff's office closed the investigation due to lack of evidence. Most evidence was returned to Spangler or destroyed.

Seven months later, Spangler married Sharon Cooper, his former lover, and they moved into the same house. They later divorced in 1988 due to marital problems. In 1990, Spangler married Donna Sundling, an aerobics instructor. They had marital problems, partly due to Donna's fear of heights. In April 1993, while hiking in the Grand Canyon, Donna fell to her death. Spangler claimed she slipped while they were taking a picture. Her death was ruled an accident. Spangler gained media attention discussing the dangers of hiking.

After Donna's death, Spangler reunited with his second wife, Sharon, who later died of a drug overdose in 1994. Spangler received $20,000 from Sharon's insurance policy. In January 1999, investigators from multiple agencies, including the FBI, linked the cold case homicides. They found Spangler's behavior suspicious and began a new investigation. A federal prosecutor with experience in capital murder cases joined the team. They approached Spangler, who was now claiming to have terminal cancer.

Spangler agreed to an interview and eventually confessed to the murders. He admitted to shooting his first wife and children to be with another woman and to pushing his third wife off a cliff in the Grand Canyon. He denied involvement in his second wife's overdose. He stated that killing was easier than divorce.

Spangler's confession led to his arrest. He was tried in federal court for the murder of his third wife and admitted to killing his first wife and children. He was sentenced to life imprisonment without parole and died in prison from cancer.