William L. Johansen was born on February 24, 1905, in Denmark. He moved to Beverly, Massachusetts, when he was very young. Johansen completed eighth grade in school. After school, he joined the Army for a short time. Later, he moved to New York City as an adult. In New York, he took on various jobs, including working in theaters and restaurants.

At one point, Johansen got into trouble for embezzling money from a restaurant. He was sentenced to three years of probation for this crime. He also worked in a hospital morgue. It was during this time that he began to have strange urges related to cutting people. These urges grew stronger when he drank whiskey. He even developed a habit of sleeping near corpses in the morgue.

Johansen later married a woman named Florence, and they had two children together.

At midnight on October 20, 1933, William L. Johansen killed his wife, Florence. After an argument, he choked her until she died. He then placed her body on a bed and cut open her abdomen with a knife to see if she was pregnant. Investigators initially ruled her death a suicide.

Johansen left New York, assuming the alias of Harry W. Gordon. He worked as a sea merchant and moved to San Francisco eighteen months later. On April 6, 1935, Johansen met Lena Betty Coffin, a 35-year-old prostitute, and took her to a motel. After procuring a room under a false name, he strangled her and taped her mouth shut to prevent her from screaming. He then stabbed her to death before cleaning the room and returning to his job.

On June 24, 1940, Johansen encountered Irene McCarthy at a beer hall. They drank together and later went to a hotel, where Johansen registered under a fake name. After McCarthy undressed and fell asleep, he strangled her with her own girdle. Johansen cut off a piece of her torso and placed it beside her body before cleaning up and leaving.

Authorities investigated Johansen after the murder of McCarthy. Witnesses identified him as the last person seen with her. He was arrested in Los Angeles on July 9, 1940. During questioning, he confessed to killing both Coffin and McCarthy, as well as revealing how he had murdered his first wife.

Johansen faced trial for the two California murders. He initially pleaded innocent but changed his plea to not guilty by reason of insanity. The court found him legally sane, and Johansen was convicted. On October 15, 1940, he was sentenced to death.

Johansen was executed on September 5, 1941, by gas chamber at San Quentin State Prison.