Richard Benjamin Speck was born in the village of Kirkwood, Illinois, the seventh of eight children of Benjamin Franklin Speck and Mary Margaret Carbaugh Speck. The family moved to Monmouth, Illinois shortly after Speck's birth. Speck and his younger sister, Carolyn, born in 1943, were much younger than their four older sisters and two older brothers (Speck's oldest brother, Robert, died at the age of 23 in an automobile accident in 1952). Speck's father worked as a packer at Western Stoneware in Monmouth and had previously worked as a farmer and logger. Speck was very close to his father, who died in 1947 from a heart attack at the age of 53. Speck was six years old at the time.

A few years later, Speck's religious, teetotaler mother fell in love with a traveling insurance salesman from Texas, Carl August Rudolph Lindberg, whom she met on a train trip to Chicago. The hard-drinking, peg-legged Lindberg, with a 25-year Criminal record that started with forgery and included several arrests for drunk driving, was the opposite of Speck's sober, hardworking father. Speck's mother married Lindberg on May 10, 1950, in Palo Pinto, Texas. Speck and his younger sister Carolyn stayed with their married sister Sara Thornton in Monmouth for a few months so Speck could finish second grade, before joining their mother and Lindberg in rural Santo, Texas, 40 miles west of Fort Worth, Texas, where Speck attended third grade.

Speck had begun drinking alcohol at age 12 and by age 15, was getting drunk almost every day. His first arrest, in 1955 at age 13, for trespassing, was followed by dozens of others for misdemeanors over the next eight years.

After a year in Santo, Speck moved with his mother, his stepfather, and his sister Carolyn to the East Dallas section of Dallas, Texas, living at ten addresses in poor neighborhoods over the next dozen years. Speck loathed his often drunk and frequently absent stepfather, who psychologically abused him with insults and threats. Speck, a poor student who needed glasses for reading but refused to wear them, struggled through Dallas public schools from fourth through eighth grade, repeating eighth grade at J. L. Long Jr. High School, in part because he refused to speak in class because of a lifelong fear of people staring at him. In autumn 1957, Speck started ninth grade at Crozier Technical High School, but failed every subject and did not return for the second semester in January 1958, dropping out just after his 16th birthday.

Speck worked as a laborer for the 7-Up bottling company in Dallas for almost three years, from August 24, 1960, to July 19, 1963. In October 1961, Speck met 15-year-old Shirley Annette Malone at the Texas State Fair. She became pregnant after three weeks of dating him. Shirley married Speck on January 19, 1962, and initially moved in with him, his mother, his sister Carolyn, and Carolyn's husband. Speck's mother and stepfather had separated, and his stepfather had moved to California. Speck stopped using the name Richard Benjamin Lindberg when he got married and began using the name Richard Benjamin Speck. When Speck's daughter, Robbie Lynn, was born on July 5, 1962, his wife did not know that Speck was serving a 22-day jail sentence for disturbing the peace in McKinney, Texas after a drunken melee.

In July 1963, Speck was caught having forged and cashed a co-worker's $44 paycheck and robbed a grocery store, stealing cigarettes, beer and $3 in cash. The 21-year-old Speck was convicted of forgery and burglary and sentenced to three years in prison. He was paroled after serving 16 months (September 16, 1963, to January 2, 1965) in the Texas State Penitentiary in Huntsville, Texas.

In December 1965 and March 1966, Nature and The Lancet published findings by British cytogeneticist Patricia Jacobs and colleagues of a chromosome survey of patients at Scotland's only security hospital for the developmentally disabled. Nine of the patients, ranging from 5 ft. 7 in. to 6 ft. 2 in. in height, were found to have an extra Y chromosome, the so-called XYY syndrome. Jacobs' hypothesis, that men with XYY syndrome are more prone to aggressive and violent behavior than males with the normal XY karyotype, was later shown to be incorrect.

In August 1966, Eric Engel, a Swiss endocrinologist and Geneticist at Vanderbilt University in Nashville, Tennessee, wrote to Speck's attorney, Cook County Public Defender Gerald W. Getty, who was reportedly planning an insanity defense. He suggested, based on Jacobs' unsubstantiated theory and Speck's 6 ft. 1 in. height, that Speck might have XYY syndrome. A chromosome analysis performed the following month by Engel revealed that Speck had a normal XY karyotype. One month later, a court-appointed panel of six Physicians rejected Getty's insanity argument and concluded that Speck was mentally competent to stand trial.

Speck's jury trial began April 3, 1967, in Peoria, Illinois, three hours southwest of Chicago, with a gag order on the press. In court, Speck was dramatically identified by the sole surviving student nurse, Cora Amurao. When Amurao was asked if she could identify the killer of her fellow students, Amurao rose from her seat in the witness box, walked directly in front of Speck and pointed her finger at him, nearly touching him, and said, "This is the man."

In a review article published in the Journal of Medical Genetics in December 1968, Michael Court Brown found no overrepresentation of XYY males in chromosome surveys of Scottish prisons and hospitals for the developmentally and mentally disabled, and suggested that any conclusions drawn from study populations composed solely of institutionalized males were likely distorted by selection bias.

In May 1969, at the annual meeting of the American Psychiatric Association, Telfer et al reported that they had found no evidence of significant behavior differences, on average, between men with XYY karyotypes and those with normal genomes, and that XYY males had been unfairly stigmatized by earlier unsupported speculation.

On June 28, 1971, the U.S. Supreme Court (citing their June 3, 1968 decision in Witherspoon v. Illinois) upheld Speck's conviction but reversed his death sentence, because more than 250 potential jurors were unconstitutionally excluded from his jury because of their conscientious or religious beliefs against capital punishment. The case was remanded back to the Illinois Supreme Court for re-sentencing.

On November 21, 1972, in Peoria, Judge Richard Fitzgerald re-sentenced Speck from 400 to 1,200 years in prison (eight consecutive sentences of 50 to 150 years). He was denied parole in seven minutes at his first parole hearing on September 15, 1976, and at six subsequent hearings in 1977, 1978, 1981, 1984, 1987, and 1990.

Speck loathed reporters, and granted only one press interview, in 1978, to Chicago Tribune columnist Bob Greene. During that interview, he publicly confessed to the murders for the first time, and said he thought he would get out of prison "between now and the year 2000," at which time he hoped to run his own grocery store Business. When Greene asked him if he compared himself to Celebrity killers like John Dillinger, he replied, "Me, I'm not like Dillinger or anybody else. I'm freakish."

Speck died of a heart attack on December 5, 1991, the eve of his 50th birthday.

In May 1996, Chicago television news anchor Bill Kurtis received video tapes made at Stateville Correctional Center in 1988 from an anonymous attorney. Showing them publicly for the first time before the Illinois state legislature, Kurtis pointed out the explicit scenes of sex, drug use, and money being passed around by prisoners, who seemingly had no fear of being caught; in the center was Speck, performing oral sex on another inmate, sharing a large quantity of cocaine with another inmate, parading in silk panties, sporting female-like breasts (allegedly grown using smuggled hormone treatments), and boasting, "If they only knew how much fun I was having, they'd turn me loose." The Illinois legislature packed the auditorium to view the two-hour video, but stopped the screening when the film showed Speck performing oral sex on another man.

Examination of Speck's brain suggested that two areas – the hippocampus, which involves memory, and the amygdala, which deals with rage and other strong emotions – encroached upon each other, their boundaries blurred. The tissue samples were lost or stolen when sent to a Boston Neurologist for further study. A Neurologist who examined photos of the tissue samples, along with results of an EEG administered to Speck in the 1960s, stated, "I have never heard of that [type of abnormality] in the history of neurology. So any abnormality that exceptional has got to have an exceptional consequence." He attributed Speck's homicidal nature to a combination of the brain abnormalities, the violence Speck suffered at the hands of his alcoholic stepfather, and his own drinking and violence in Texas.

Felony Court Judge Herbert J. Paschen appointed an impartial panel to report on Speck's competence to stand trial and his sanity at the time of the crime — a panel of three Physicians suggested by the defense and three Physicians selected by the prosecution, consisting of five Psychiatrists and one general surgeon. The panel's confidential report deemed Speck competent to stand trial and concluded he had not been insane at the time of the murders.

Speck spent the rest of the day drinking in nearby taverns before accosting, at knifepoint, Ella Mae Hooper, a 53-year-old woman who had spent the day drinking at the same taverns as Speck. Speck took her to his room at the Shipyard Inn, raped her, and stole her black $16 mail-order .22 caliber Röhm pistol. After dinner at the nearby Kay's Pilot House, Speck returned to drink at the Shipyard Inn’s tavern until 10:20 p.m., when he left dressed entirely in black, armed with a switchblade and Ella Mae Hooper's handgun, and walked a mile and a half west on E. 100th St. to the nurses' townhouse at 2319 E. 100th St.