Shawcross was born in Kittery, Maine, the first of four children of Arthur Roy Shawcross and Elizabeth "Bessie" ("Betty") Yerakes Shawcross. His family moved to Watertown in New York State when he was young. While several later tests showed Shawcross' intelligence to be sub-normal or even "borderline retarded", he received As and Bs in his first two years of grade school, but was later tested to have an I.Q. of 86, signifying below average intelligence. However, when tested in the army, Shawcross scored just above average in intelligence tests, scoring 105 upon entry and 108 at discharge. This was also the case with the intelligence tests he underwent while on trial in 1990, scoring 107.
Shawcross said throughout his childhood, he was a frequent bed-wetter. He later claimed his mother would insert foreign objects into his rectum, that his mother performed oral sex on him when he was 9, and that during junior high school he had sexual relations with his sister. Shawcross had a reputation at school as a bully and would frequently act out violently. He dropped out of high school in 1960.
In April 1967 he was drafted by the Army at age 21. At this time, Shawcross divorced his first wife and gave up the rights to their 18-month-old son, whom he never saw again. He served one tour of duty in Vietnam, where he boasted of grotesque combat exploits, such as "beheading mama-sans and nailing their heads to trees as a warning to the Vietcong" — though in fact he never saw combat. After Vietnam he was stationed at Fort Sill in Lawton, Oklahoma as an armorer. His second wife Linda experienced several aspects of his disturbing behavior, especially his penchant for starting fires; an Army psychiatrist told her that Shawcross derived "sexual enjoyment" from fire starting.
After his discharge from the Army, Shawcross moved with his wife from Oklahoma to Clayton, New York. His wife would soon divorce him, and he began committing crimes such as arson and burglary. His offenses earned him a five-year sentence at Attica Correctional Facility, and later Auburn Correctional Facility. After serving 22 months he was granted early release in October 1971, in part due to his role in the rescue of a prison guard during a riot.
Shawcross returned to Watertown, eventually getting a job with the Watertown Public Works Department, and marrying for a third time. On May 7, 1972, he raped and killed 10-year-old Jack Owen Blake (Oct. 18, 1961 - May 7, 1972), his first known victim, after luring the boy into some woods in Watertown, New York. Four months later, on September 2, he raped and killed eight-year-old Karen Ann Hill (1964 - Sep. 20, 1972), who had been visiting Watertown with her mother for the Labor Day weekend. Arrested in October, he confessed to both killings. Under the terms of a plea bargain he agreed to reveal the location of Blake's body; in return he was permitted to plead guilty to a lesser charge of manslaughter in the Hill case, all other charges were dropped, and he received a 25-year sentence at Green Haven Correctional Facility.
After twelve years, inexperienced prison staff and social workers concluded that Shawcross was "no longer dangerous", disregarding the warnings of psychiatrists, who had assessed Shawcross as a "schizoid psychopath". He was released on parole in April 1987. He had difficulty settling down in communities as the neighbors would protest his presence and employers would fire him. He first moved into Binghamton, New York, then relocated to Delhi, New York, with his girlfriend, Rose Marie Walley. When Delhi residents became aware of Shawcross' presence, the couple moved to nearby Fleischmanns, New York, only to be met with hostility there as well. Finally, in late June 1987, Shawcross' parole officer moved him and Walley into a transient hotel in Rochester, New York, but failed to notify Rochester authorities of this action. In mid-October Shawcross and Walley found more permanent lodgings at 241 Alexander Street in Rochester.
In March 1988, Shawcross began murdering again, primarily prostitutes in the area (apart from June Stott, who was a local and was the first one of his victims to be mutilated after her death), before his capture less than two years later. He was convicted of 11 murders, with a 12th not officially charged to him. The victims were as follows:
All the victims were murdered in Monroe County, except for Gibson, who was killed in neighboring Wayne County. The retired detective Robert Keppel has argued that the detectives investigating the case over-relied on the concept of modus operandi, at times searching for multiple suspects due to small differences in the profiles of each victim.
June Cicero's body was discovered by aerial surveillance on January 3, 1990.
Shawcross was spotted by the surveillance team (and by an eyewitness) standing near his car, apparently urinating, on a bridge over Salmon Creek; upon whose frozen waters the body of his final victim was dumped.
In November 1990, Shawcross was tried by Monroe County First Assistant District Attorney Charles J. Siragusa for the 10 murders in Monroe County. Shawcross pleaded not guilty by reason of insanity, with testimony from psychiatrist Dorothy Lewis that he suffered from brain damage, multiple personality disorder and post-traumatic stress disorder, and had been sexually abused as a child. Shawcross, who had served in Vietnam with the 4th Supply and Transport Company of the 4th Infantry Division, had told many outlandish tales of murderous activities (including cannibalism), often perpetrated while alone in the jungle. From the time Shawcross returned from his tour of duty, he told acquaintances of seeing American soldiers "skinned from their neck to their ankles", and claimed to have decapitated two women he had victimized, "placing" their heads on poles. FBI criminal profiler Robert K. Ressler reviewed the PTSD claim on behalf of the prosecution before the trial. Ressler wrote that "his claim of having witnessed wartime atrocities was patently outrageous and untrue." Prosecution psychiatrist Dr. Park Dietz said Shawcross had antisocial personality disorder.
Shawcross was held at the Sullivan Correctional Facility in Fallsburg, New York, until he died on November 10, 2008 at the Albany Medical Center.
In 2003, Shawcross was interviewed by British reporter Katherine English for a documentary on cannibalism. Shawcross bragged about slicing out and eating the vulvas of three victims, but refused to discuss his earlier claim of eating the genitals of his first victim, Jack Blake.
Officials said Shawcross complained of a pain in his leg on the afternoon of November 10, 2008, his date of death. He was taken to Albany Medical Center, where he went into cardiac arrest. Shawcross died at 9:50 p.m. He was cremated.