Joubert was born on July 2, 1963, in Lawrence, Massachusetts. He had one sister, and when he was six years old, in 1969, his parents divorced. In 1974, Joubert and his sister moved with their mother to a rundown apartment in Portland, Maine, to start anew. Despite his wishes, he wasn't allowed to visit his father and grew to resent his controlling mother. Rich Pitre, his high school band teacher, friend, and mentor, recalled, “My impression was that the mother was cold and extremely manipulative. He was under a very, very tight leash.”

Academically, Joubert was an honor roll student and highly driven, but he faced bullying from some of his peers. Francesca Bergan, his former Spanish teacher, described him as "a 'little boy' all through his high school years." She noted that although many kids are picked on, they often defend themselves. However, she felt that Joubert didn't defend himself and might have believed he deserved the abuse, which included shoving, name-calling, and teasing.

To cope with his feelings of isolation, Joubert became more involved in extracurricular activities. He played the clarinet in the marching band, ran on the school track team, and joined the Cub Scouts. However, around this time, his sadistic and homicidal fantasies intensified. He began to imagine killing strangers or restraining and gagging those who resisted him. A later psychiatric report described him deriving pleasure from the despair of his victims, quoting him as saying, “if you are going to do it, get it over with.”

At the age of 13, Joubert's violent tendencies escalated. He stabbed a young girl with a pencil, feeling sexually stimulated by her cry of pain. The following day, he slashed another girl with a razor blade while biking past her. He was never identified or charged for these offenses. In another incident, he beat up a boy and nearly strangled him to unconsciousness. Having been bullied himself, Joubert relished the physical and mental power he felt from these acts of dominance and began to casually consider stabbing or slashing others. Despite these troubling behaviors, Joubert graduated from Cheverus High School, a Catholic secondary school in Portland, in 1981.

On August 22, 1982, 11-year-old Richard "Ricky" Stetson left his home to go jogging on the Back Cove Trail in Portland, Maine. When he didn't return by dark, his worried parents contacted the police. The following day, a motorist discovered Ricky's body on the side of Interstate 295. He had been attacked, undressed, stabbed, strangled, and bitten. An initial suspect was arrested, but the bite marks did not match, leading to his release after a year and a half in custody. The case went cold until January 1984.

On September 18, 1983, 13-year-old Danny Joe Eberle vanished while delivering newspapers in Bellevue, Nebraska. His bicycle and undelivered newspapers were found, but there was no sign of a struggle. A subsequent investigation revealed that Eberle had been abducted, bound, and tortured before his body was found three days later, four miles from where he disappeared. The FBI got involved due to the kidnapping.

Despite various leads, including the arrest of a young man who failed a polygraph test but didn't fit the FBI profile, the case stalled. Known pedophiles in the area were questioned without success.

On December 2, 1983, 12-year-old Christopher Walden disappeared in Papillion, Nebraska, near where Eberle's body had been found. Witnesses reported seeing a man in a tan car. Joubert later confessed to abducting Walden, who resisted but was overpowered and brutally murdered. Walden’s body was found two days later, five miles from town, with significant differences in the crime scene compared to Eberle’s case.

On January 11, 1984, a preschool teacher reported a suspicious young man driving in the area. When the driver saw her noting his license plate, he threatened her and fled. The car was traced to John Joubert, a radar technician from Offutt Air Force Base. Rope matching that used to bind Eberle was found in Joubert’s room, linking him to the crime.

Robert K. Ressler, the FBI's head profiler, connected Joubert to a similar unsolved case in Maine. Bite mark comparisons confirmed Joubert's guilt in the murder of Ricky Stetson. Investigations revealed Joubert had a violent history in Maine, including the stabbing of a girl in 1979 and attacks on two others in 1980.

Joubert confessed to the Nebraska murders and, after initially pleading not guilty, changed his plea to guilty. Psychiatric evaluations indicated he had obsessive-compulsive disorder, sadistic tendencies, and schizoid personality disorder, but he was not found psychotic at the time of the crimes. He was sentenced to death for the Nebraska murders and to life imprisonment in Maine for Stetson's murder.

In 1995, Joubert challenged his death sentences, arguing the term "exceptional depravity" was too vague. The district court agreed, but the decision was overturned on appeal, and his sadistic behavior was confirmed. Joubert was executed in Nebraska on July 17, 1996. In his final statement, he apologized to the victims' families and the people of Nebraska, asking for forgiveness. His last meal included pizza, strawberry cheesecake, and black coffee.