Hadden Irving Clark was born on July 31, 1952, in Troy, New York. He was the second of four children in his family. Growing up, Clark experienced a challenging home life. Both of his parents struggled with alcoholism and often fought in front of their children. His mother, when drunk, would dress him in girls' clothes and call him "Kristen." This behavior affected Clark deeply. His father eventually committed suicide, which added to the family's struggles.

As a teenager, Clark faced bullying from other kids. In response, he began to torture and kill animals that belonged to the children who bullied him. This behavior set a concerning pattern in his life. Despite these early challenges, he trained as a chef and joined the United States Navy. However, his time in the Navy ended when he was discharged due to a diagnosis of paranoid schizophrenia.

After leaving the Navy, Clark held various menial jobs but often found himself homeless. He faced legal issues as well, getting arrested multiple times for theft and retaliation. One of these arrests was for robbery after he vandalized a former landlord's property and stole items from it.

On May 31, 1986, Hadden Clark committed his first known murder. He killed a six-year-old girl named Michele Dorr, who had come to his brother's house looking for her friend. Clark lured Michele to an upstairs room and fatally attacked her. After the murder, he engaged in gruesome acts with her body before hiding it in a duffel bag. He buried her in a park located 12 miles away from the house.

On October 18, 1992, Clark murdered a 23-year-old woman named Laura Houghteling in Bethesda, Maryland. At the time, he was working as a gardener for Laura's mother. After being accused of stealing tools, Clark entered the house secretly and killed Laura in her bedroom. He used a kitchen knife to stab her and suffocated her with a pillow. He then moved her body in a bedsheet through the woods and buried it a short distance away. In his haste, he left behind a pillowcase with his fingerprint. Later, Clark returned to the scene dressed in a wig and women’s clothing to mislead others into believing Laura had left alive.

After an extensive investigation, police found evidence linking Clark to both murders. He confessed and guided detectives to Laura’s body, which he had buried eight months earlier. In 2000, police found evidence in his brother's old house that connected him to Michele Dorr's murder. Clark revealed the location of her body as well, which was discovered in January 2000.

Clark has claimed to have murdered many more people throughout his life. He wrote a letter in 2004 asserting that he killed a woman in Massachusetts in 1974, calling her the “Lady of the Dunes.” However, police have doubts about these claims due to questions surrounding his mental health.

In December 2000, Clark also led police to his grandparents' former property, where they found a bucket filled with over 200 pieces of jewelry that he referred to as "trophies" from his victims. This evidence reinforced suspicions that there were more unidentified victims connected to him.