Johannes Opdam was born on October 30, 1916, in Surabaya, which was part of the Dutch East Indies. He spent his early years in this region before moving to the Netherlands in 1936. In the Netherlands, he pursued a career in medicine and worked hard to become a doctor.

While studying, Johannes met Arnolda van Eyl, who was the daughter of a teacher. They got married, starting a new life together. Arnolda was devoted to her faith and family.

During his wife's vacation in 1951, Johannes Opdam had an affair with their maid, Nellie. When Mrs. Opdam found out, she did not seek a divorce due to her strong Catholic beliefs. Instead, she fired Nellie. Despite this, Opdam continued the affair. In 1952, Mrs. Opdam's health worsened and she died on September 24. Opdam, a physician, claimed she had a brain tumor. However, an autopsy suggested an "unknown" cause of death. A second examination of her body found prussic acid, which led to Opdam's arrest. During his trial, he even blamed his mother-in-law for the murder.

On June 8, 1954, Opdam was sentenced to life in prison. While in prison in Leeuwarden, he met another inmate named Arie Lodder, who had also killed his wife. They decided to write notes confessing to each other's murders, which they planned to use if one of them died. In February 1958, Lodder was found dead in his cell, which was next to Opdam's. He had also been poisoned with prussic acid. A note was discovered with Lodder stating he had taken medicine prescribed by Opdam.

Opdam obtained the poison from his cousin-in-law. There were also rumors that Lodder's parents sent the poison to the prison, thinking it was medicine for their son. Despite the letters and attempted set-up, the plan did not work. In 1961, Opdam was again sentenced to life in prison for the murder of Arie Lodder. He remains notable for being the only person in Dutch history to receive two life sentences for two different murders.