On February 15, the court reconvened to hear the verdict: Dahmer was ruled to be sane and not suffering from a mental disorder at the time of each of the 15 murders for which he was tried, although in each count, two of the 12 jurors signified their dissent. On the first two counts, Dahmer was sentenced to life imprisonment plus ten years, with the remaining 13 counts carrying a mandatory sentence of life imprisonment plus 70 years. The death penalty was not an option for Judge Gram to consider at the penalty phase as the State of Wisconsin had abolished capital punishment in 1853.

Less than three months after the murder of Smith, Dahmer encountered a 22-year-old Chicago native named Ernest Miller on the corner of North 27th Street. Miller agreed to accompany Dahmer to his apartment for $50 and further agreed to allow him to Listen to his heart and stomach. When Dahmer attempted to perform oral sex upon Miller, he was informed: "That'll cost you extra," whereupon Dahmer gave his intended victim a drink laced with two sleeping pills. On this occasion, however, he had had only two sleeping pills to give his victim; therefore, he killed Miller by slashing his carotid artery with the same knife he used to dissect his victims' bodies. Miller bled to death within minutes. Dahmer then posed the nude body for various suggestive Polaroid photographs before placing the body in his bathtub for dismemberment. Dahmer repeatedly kissed and talked to the severed head while he dismembered the remainder of the body. He wrapped Miller's heart, biceps, and portions of flesh from the legs in plastic bags and placed them in the fridge for later consumption. Dahmer boiled the remaining flesh and organs into a "jelly-like substance" using Soilex, which again enabled him to rinse the flesh off the skeleton, which he intended to retain. To preserve the skeleton, he placed the bones in a light bleach solution for 24 hours before allowing them to dry upon a cloth for one week; the severed head was initially placed in the fridge before also being stripped of flesh, then painted and coated with enamel.

In the early morning hours of May 27, Dahmer returned toward his apartment to discover Sinthasomphone sitting naked on the corner of 25th and State, talking in Laotian, with three distressed young women standing near him. Dahmer approached the trio and explained to the women that Sinthasomphone (whom he referred to by an alias) was his friend, and attempted to lead him to his apartment by the arm. The three women dissuaded Dahmer, explaining they had phoned 911. Upon the arrival of two officers named John Balcerzak and Joseph Gabrish, Dahmer's demeanor relaxed: he informed the officers that Sinthasomphone was his 19-year-old boyfriend, that he had drunk too much following a quarrel, and that he frequently behaved in this manner when intoxicated. The three women were exasperated and when one of the trio attempted to indicate to one of the officers that Sinthasomphone was bleeding from his buttocks and that he had seemingly struggled against Dahmer's attempts to walk him to his apartment, the officer harshly informed her to "butt out," "shut the hell up" and to not interfere, adding the incident was "domestic."

Jeffrey Dahmer was born at the Evangelical Deaconess Hospital in Milwaukee, Wisconsin, on May 21, 1960, the first of two sons born to Joyce Annette (née Flint) and Lionel Herbert Dahmer. Dahmer's mother worked as a teletype machine instructor, and his father was a student at Marquette University, working towards a degree in chemistry. His father was of German and Welsh ancestry, and his mother was of Norwegian and Irish ancestry.

The Dahmer family relocated to Doylestown, Ohio, in October 1966. At the time, Joyce Dahmer was pregnant with her second child. When she gave birth to a baby boy on December 18, 1966, Jeffrey was allowed to choose the name of the baby. He chose the name David for his younger brother. The same year, Lionel Dahmer achieved his degree and subsequently obtained employment as an analytical Chemist in the city of Akron.

In 1968 the family relocated to Bath, Ohio. Two years later, over a family meal of chicken, Dahmer asked his father what would happen if the bones of the chicken were to be placed in a bleach solution. Lionel Dahmer was, by this stage, concerned as to his elder son's placid and lethargic attitude and his solitary existence; therefore, he was delighted at the initiative displayed by his son towards what he believed to be scientific curiosity. He willingly demonstrated to his son how to safely bleach and (later) preserve animal bones. This knowledge regarding the cleansing and preserving of bones was used by Dahmer on many of the animal remains which he continued to avidly collect.

Jeffrey Dahmer is known to have killed 17 young men between 1978 and 1991. Of these victims, 12 were killed in his North 25th Street apartment. Three further victims were murdered and dismembered at his grandmother's West Allis residence, with his first and second victims being murdered at his parents' home in Bath, Ohio, and at the Ambassador Hotel in Wisconsin respectively. A total of 14 of Dahmer's victims were from various ethnic minority backgrounds, with nine victims being black. Dahmer was Adam Ant that the race of his victims was incidental to him and that it was the body form of a potential victim that attracted his attention.

In January 1979, on his father's urging, Dahmer enlisted in the U.S. Army, where he trained as a medical specialist at Fort Sam Houston before July 13, 1979, when he was stationed in Baumholder, West Germany, where he served as a combat medic in 2nd Battalion, 68th Armored Regiment, 8th Infantry Division. According to published reports, in Dahmer's first year of Service, he was an "average or slightly above average" soldier. Two Soldiers attest to having been raped by Dahmer while in the army; one of whom stated in 2010 that while stationed at Baumholder, Dahmer had repeatedly raped him over a 17-month period, while another soldier believes he was drugged, then raped by Dahmer inside an armored personnel carrier in 1979. Owing to Dahmer's alcohol abuse, his performance deteriorated and, in March 1981, he was deemed unsuitable for military Service and was later discharged from the Army. He received an honorable discharge, as his superiors did not believe that any problems Dahmer had in the Army would be applicable to civilian life.

After his return to Ohio, Dahmer initially resided with his father and stepmother and insisted on being delegated numerous chores to occupy his time while he looked for work. However, he continued to drink heavily and, just two weeks after his return, Dahmer was arrested for drunk and disorderly conduct, for which he was fined $60 and given a suspended ten-day jail sentence. Dahmer's father tried unsuccessfully to wean his son off alcohol. In December 1981, Dahmer's father and stepmother sent him to live with his grandmother in West Allis. Dahmer's grandmother was the only family member to whom he displayed any affection; they hoped that her influence, plus the change of scenery, might inspire Dahmer to refrain from alcohol, find a job, and live responsibly. Initially, Dahmer's living arrangements with his grandmother were harmonious: he accompanied her to church; willingly undertook chores; actively sought work; and abided by most of her house rules (although he did continue to drink and smoke). This new influence in his life initially brought results and, in early 1982, Dahmer found employment as a phlebotomist at the Milwaukee Blood Plasma Center. He held this job for a total of 10 months before being laid off. He remained unemployed for over two years, during which he lived upon whatever money his grandmother gave him.

Shortly before losing his job, Dahmer was arrested for indecent exposure. On August 7, 1982, at Wisconsin State Fair Park, Dahmer was observed to expose himself to a crowd of 25 women and children. For this incident, he was convicted and fined $50 plus court costs.

By late 1985, he had begun to regularly frequent the bathhouses, which he later described as being “relaxing places”, but during his sexual encounters, he became frustrated at his partners' moving during the sexual act. Following his arrest, he stated: "I trained myself to view people as objects of pleasure instead of [as] people". For this reason, from June 1986, he began to administer sleeping pills to his partners, giving them liquor laced with the sedatives, then raping their unconscious bodies. After approximately 12 such instances, the bathhouses' administration revoked Dahmer's membership, and he began to use hotel rooms to continue this practice. Shortly after his membership of the bathhouses was revoked, Dahmer read a report in a newspaper regarding the upcoming funeral of an 18-year-old male; he conceived the idea of stealing the freshly interred corpse and taking it home. According to Dahmer, he attempted to dig the coffin from the ground, but found the soil too hard, before abandoning the plan.

In November 1987, Dahmer—at the time residing with his grandmother in West Allis—encountered a 25-year-old man from Ontonagon, Michigan named Steven Tuomi at a bar and persuaded him to return to the Ambassador Hotel, where Dahmer had rented a room for the evening. According to Dahmer, he had no intention of murdering Tuomi, but simply intended to drug and rape him as he lay unconscious. The following morning, however, he awoke to find Tuomi lying beneath him on the bed, his chest "crushed in" and "black and blue" with bruises. Blood was also seeping from the corner of his mouth, and Dahmer's fists and one forearm were extensively bruised. Dahmer stated he had absolutely no memory of having killed Tuomi, and later informed investigators that he simply "could not believe this had happened." To dispose of Tuomi's body, he purchased a large suitcase in which he transported the body to his grandmother's residence. There, one week later, he severed the head, arms, and legs from the torso, then filleted the bones from the body before cutting the flesh into pieces small enough to handle. He then placed the flesh inside plastic garbage bags. He wrapped the bones inside a sheet and pounded them into splinters with a sledgehammer. The entire dismemberment process took Dahmer approximately two hours to complete, and all of Tuomi's remains—excluding the severed head—were disposed of in the trash.

In September 1988, Dahmer's grandmother asked him to move out of her house because of his habit of bringing young men to her house late at night and the foul smells emanating from both the basement and the garage. Dahmer found a one-bedroom apartment on North 25th Street and moved into his new residence on September 25. The following day, Dahmer was arrested for drugging and sexually fondling a 13-year-old boy whom he had lured to his home on the pretext of posing nude for photographs. In January 1989, Dahmer was convicted of second-degree sexual assault and of enticing a child for immoral purposes. Sentencing for the assault was suspended until May 1989. On March 20, Dahmer commenced a ten-day Easter absence from work, during which he moved back into his grandmother's home.

Two months before his scheduled release from the work camp, Dahmer was paroled from this regime. (His five years' probation imposed in 1989 began at this point.) On release, Dahmer temporarily moved back to his grandmother's home in West Allis before, in May 1990, moving into the Oxford Apartments, located on North 25th Street in Milwaukee. Although located in a high crime area, the apartment was close to his workplace, was furnished and, at $300 a month inclusive of all bills excluding electricity, was economical.

Most of Dahmer's victims were killed by strangulation after being drugged with sedatives, although his first victim was killed by a combination of bludgeoning and strangulation and his second victim was battered to death, with one further victim killed in 1990, Ernest Miller, dying of a combination of shock and blood loss due to his carotid artery being cut. Many of Dahmer's victims killed in 1991 had holes bored into their skulls through which Dahmer injected hydrochloric acid or, later, boiling water, directly into the brain in an attempt to render a permanent, submissive, unresistant state. On at least three occasions, this proved fatal although on none of these occasions was this Dahmer's intention.

On August 5, 1991, a candlelight vigil to celebrate and heal the Milwaukee community was attended by more than 400 people. Present at the vigil were community Leaders, gay rights Activists, and family members of several of Dahmer's victims. Organizers stated the purpose of the vigil was to enable Milwaukeeans to "share their feelings of pain and anger over what happened".

The Oxford Apartments at 924 North 25th Street, where Dahmer had killed 12 of his victims, were demolished in November 1992. The site is now a vacant lot. Alternate plans to convert the site into either a memorial garden, a playground, or to reconstruct new housing have failed to materialize.

Lionel Dahmer is retired and now lives with his second wife, Shari. Both have refused to change their surname and have professed their love of Jeffrey in spite of his crimes. In 1994, Lionel published a book, A Father's Story, and donated a portion of the proceeds from his book to the victims' families. Most of the families showed support for Lionel and Shari, although three families subsequently sued Lionel Dahmer: two for using their names in the book without obtaining prior consent; and a third family—that of Steven Hicks—filing a wrongful death suit against Lionel Dahmer, Shari, and former wife Joyce, citing parental negligence as the cause for the claim.

Dahmer had stated in his will he wished for no services to be conducted and that he wished to be cremated. In September 1995, Dahmer's body was cremated, and his ashes divided between his parents.

Dahmer's estate was awarded to the families of 11 of his victims who had sued for damages. In 1996, Thomas Jacobson, a Lawyer representing eight of the families, announced a planned auction of Dahmer's estate. Although victims' relatives stated the motivation was not greed, the announcement sparked controversy. A civic group, Milwaukee Civic Pride, was quickly established in an effort to raise the funds to purchase and destroy Dahmer's possessions. The group pledged $407,225, including a $100,000 gift by Milwaukee real estate developer Joseph Zilber, for purchase of Dahmer's estate; five of the eight families represented by Jacobson agreed to the terms, and Dahmer's possessions were subsequently destroyed and buried in an undisclosed Illinois landfill.

Joyce Flint died of cancer in November 2000. Prior to her death, she had attempted suicide on at least one occasion. Jeffrey's younger brother, David, changed his surname and lives in anonymity.

At 11:30 p.m. on July 22, Edwards flagged down two Milwaukee police officers at the corner of North 25th Street. The officers noted Edwards had a handcuff attached to his wrist, whereupon Edwards explained to the officers that a "freak" had placed the handcuffs upon him and asked if the police could remove them. When the officers' handcuff keys failed to fit the brand of handcuffs, Edwards agreed to accompany the officers to the apartment where, Edwards stated, he had spent the previous five hours before escaping. When the officers and Edwards arrived at Apartment 213, Dahmer invited the trio inside and acknowledged he had indeed placed the handcuffs upon Edwards, although he offered no explanation as to why he had done so. At this point, Edwards divulged to the officers that Dahmer had also brandished a large knife upon him and that this had happened in the bedroom. Dahmer made no comment to this revelation; indicating to one of the officers, Rolf Mueller, that the key to the handcuffs was in his bedside dresser in the bedroom. As Mueller entered the bedroom, Dahmer attempted to pass Mueller to himself retrieve the key, whereupon the second officer present, Robert Rauth, informed him to "back off".

The trial lasted two weeks. On February 14, both counsels delivered their closing arguments to the jury. Each counsel was allowed to speak for two hours. Defense attorney Gerald Boyle argued first. Repeatedly harking to the testimony of the mental health professionals—almost all of whom had agreed Dahmer was suffering from a mental disease—Boyle argued that Dahmer's compulsive killings had been a result of "a sickness he discovered, not chose." Boyle portrayed Dahmer as a desperately lonely and profoundly sick individual "so out of control he could not conform his conduct any more." Following the defense counsel's 75-minute closing argument, Michael McCann delivered his closing argument for the prosecution, describing Dahmer as a sane man, in full control of his actions, who simply strove to avoid detection. McCann argued that the act of murder was committed in hostility, anger, resentment, frustration, or hatred, and that the 15 victims for whose murder he was tried "died merely to afford Dahmer a period of sexual pleasure." McCann further argued that by pleading guilty but insane to the charges, Dahmer was seeking to escape responsibility for his crimes.

On July 15, Dahmer encountered 24-year-old Oliver Lacy at the corner of 27th and Kilbourn. Lacy agreed to Dahmer's ruse of posing nude for photographs and accompanied him to his apartment, where the pair engaged in tentative sexual activity before Dahmer drugged Lacy. On this occasion, Dahmer intended to prolong the time he spent with Lacy while alive; after unsuccessfully attempting to render Lacy unconscious with chloroform, he phoned his workplace to request a day's absence; this was granted, although the next day, he was suspended. After strangling Lacy, Dahmer had sex with the corpse before dismembering him. He placed Lacy's head and heart in the refrigerator and his skeleton in the freezer. Four days later, on July 19, Dahmer received word that he was fired. Upon receipt of this news, Dahmer lured 25-year-old Joseph Bradehoft to his apartment. Bradehoft was strangled and left lying on Dahmer's bed covered with a sheet for two days. On July 21, Dahmer removed these sheets to find the head covered in maggots, whereupon he decapitated the body, cleaned the head and placed it in the refrigerator. He later acidified Bradehoft's torso along with those of two other victims killed within the previous month.