Lyle Richard Brummett was born on December 26, 1956, in Kerrville, Texas. Information about his early life is limited. Brummett stated that he dropped out of high school because he had little interest in studying and did not get along with his teachers. After leaving school, he found work driving trucks for lumberyards. Over the years, he developed interests in activities such as sailing, camping, motorcycles, and fishing.

Brummett claimed that he had his first emotional outburst around 1974 when he was with a girl. Following this incident, he visited several psychiatrists but said that they were unable to help him. His first known criminal act was an arrest for burglary in July 1975, where he used the name "Lyle Richard Stone." Although he was released after this arrest, he was later re-arrested for two rapes and credit card abuse. He was released after posting a bond of $17,500.

On September 17, 1975, Lyle Richard Brummett, along with his friend Allen Ladd Woody, picked up two girls—18-year-old Carol Ann London and 16-year-old Elizabeth "Beth" Pearson—whose car had broken down near Kerrville, Texas. They drove the girls to an isolated pasture about 12 miles from Kerrville. There, they assaulted and strangled both girls. After committing the crimes, Brummett and Woody left the victims' bodies under an oak tree. The girls’ car was later found abandoned in a parking lot, but their bodies were not discovered until after Brummett's arrest.

A few months later, on August 15, 1976, Brummett was at the home of 22-year-old Dianne Kathleen Roberts, whom he knew. He entered her house by loosening a window screen and then assaulted and killed her. He covered her body with a pillowcase and left it on her bed. Roberts' body was found by her boyfriend when he returned home from a trip.

Brummett was arrested the day after Roberts' murder. While in custody, he admitted to police that he and Woody had killed London and Pearson. They followed him to the location where he revealed their bodies had been left. Both Brummett and Woody were charged with the murders and placed in separate jails as they awaited trial.

While in jail, Brummett was involved in another crime, where he and several inmates assaulted a fellow inmate. He also planned an escape from jail, managing to slip away during a search for the body of another victim. After his escape, he contacted police himself and asked to be taken back into custody.

In April 1977, during the trial against Allen Woody, Brummett testified, claiming Woody was responsible for the death of one of the girls while admitting he killed the other. As part of a deal, he was not charged with the murder of Pearson. Woody was convicted and sentenced to 99 years in prison.

After the trials, Brummett was sentenced to life in prison for the murders of both London and Roberts. He has been serving his sentence at the Ramsey Unit in Rosharon, Texas, and has applied for parole multiple times but has been denied each time. The families of the victims have consistently opposed his release.