Gary Leon Ridgway was born on February 18, 1949, in Salt Lake City, Utah. He was the second of Mary and Thomas Ridgway's three sons. His home life was somewhat troubled; relatives have described his mother as domineering and have said that young Ridgway witnessed more than one violent argument between his parents. His father was a bus driver who would often complain about the presence of sex workers.

Ridgway graduated from Tyee High School in 1969 and married his 19-year-old high school girlfriend, Claudia Kraig. He joined the U.S. Navy and was sent to Vietnam, where he served on board a supply ship and saw combat. During his time in the military, Ridgway began to have frequent sexual intercourse with numerous sex workers and contracted gonorrhea; although angered by this, he continued his practice without protection. While Ridgway was abroad, Kraig had an extramarital affair. The marriage ended within a year.

Ridgway began each murder by picking up a woman, usually a prostitute. He sometimes showed the woman a picture of his son, to trick her into trusting him. After raping her, Ridgway strangled her from behind. He initially strangled them manually. However, many victims inflicted wounds and bruises on his arm while trying to defend themselves. Concerned these wounds and bruises would draw attention, Ridgway began using ligatures. He killed most victims in his home, his truck, or a secluded area. In the early 1980s, the King County Sheriff's Office formed the Green River Task Force to investigate the murders. The most notable members of the task force were Robert Keppel and Dave Reichert, who periodically interviewed incarcerated serial killer Ted Bundy in 1984. Bundy offered his opinions on the psychology, motivations, and behavior of the killer; he suggested that the killer was revisiting the dump sites to have sex with his victims, which turned out to be true, and if police found a fresh grave, they should stake it out and wait for him to come back. Also contributing to the investigation was John E. Douglas, who has since written much on the subject of the Green River Killer.

Ridgway was arrested in 1982 and 2001 on charges related to prostitution. He became a suspect in the Green River killings in 1983. In 1984, Ridgway took and passed a polygraph test (quality control protocols later developed in the FBI after careful review determined that Ridgway actually failed his polygraph test), and on April 7, 1987, police took hair and saliva samples from Ridgway. Around 1985, Ridgway began dating Judith Mawson, who became his third wife in 1988. Mawson claimed in a 2010 television interview that when she moved into his house while they were dating, there was no carpet. Detectives later told her he had probably wrapped a body in the carpet. In the same interview, she described how he would leave for work early in the morning some days, ostensibly for the overtime pay. Mawson speculated that he must have committed some of the murders while supposedly working these early morning shifts. She claimed that she had not suspected Ridgway's crimes before she was contacted by authorities in 1987, and had not even heard of the Green River Killer before that time because she did not watch the news.

The samples collected in 1987 were later subjected to a DNA analysis, providing the evidence for his arrest warrant. On November 30, 2001, Ridgway was at the Kenworth Truck factory, where he worked as a spray Painter, when police arrived to arrest him. Ridgway was arrested on suspicion of murdering four women nearly 20 years earlier after first being identified as a potential suspect, when DNA evidence conclusively linked semen left in the victims to the saliva swab taken by the police. The four victims named in the original indictment were Marcia Chapman, Opal Mills, Cynthia Hinds, and Carol Ann Christensen. Three more victims—Wendy Coffield, Debra Bonner, and Debra Estes—were added to the indictment after a forensic scientist identified microscopic spray paint spheres as a specific brand and composition of paint used at the Kenworth factory during the specific time frame when these victims were killed.

At the time of his December 18, 2003 sentencing, authorities had been able to find at least 48 sets of remains, including victims not originally attributed to the Green River Killer. Ridgway was sentenced for the deaths of each of these 48 victims, with a plea agreement that he would "plead guilty to any and all Future cases (in King County) where his confession could be corroborated by reliable evidence."

Ridgway confessed to more confirmed murders than any other American serial killer. Over a period of five months of police and prosecutor interviews, he confessed to 48 murders—42 of which were on the police's list of probable Green River Killer victims. On February 9, 2004, county prosecutors began to release the videotape records of Ridgway's confessions. In one taped interview, he told investigators initially that he was responsible for the deaths of 65 women, but in another taped interview with Reichert on December 31, 2003, Ridgway claimed to have murdered 71 victims and confessed to having had sex with them before killing them, a detail which he did not reveal until after his sentencing. In his confession, he acknowledged that he targeted prostitutes because they were "easy to pick up" and that he "hated most of them." He confessed that he had sex with his victims' bodies after he murdered them, but claimed he began burying the later victims so that he could resist the urge to commit necrophilia.

On November 23, 2005, The Associated Press reported that a weekend hiker found the skull of one of the 48 women Ridgway admitted murdering in his 2003 plea bargain with King County prosecutors. The skull of another victim, that of Tracy Winston who was 19 when she disappeared from Northgate Mall on September 12, 1983, was found on November 20, 2005 by a man hiking in a wooded area near Highway 18 near Issaquah, southeast of Seattle.

Ridgway is suspected of—but not charged with—murdering the remaining six victims of the original list attributed to the Green River Killer. In each case, either Ridgway did not confess to the victim's death, or authorities have not been able to corroborate their suspicion with reliable evidence.

Ridgway was placed in solitary confinement at Washington State Penitentiary in Walla Walla in January 2004. On May 14, 2015, he was transferred to the USP Florence, a high-security federal prison east of Cañon City, Colorado. In September 2015, after a public outcry and discussions with Governor Jay Inslee, Corrections Secretary Bernie Warner announced that Ridgway would be transferred back to Washington to be "easily accessible" for open murder investigations. Ridgway was returned by chartered plane to Washington State Penitentiary in Walla Walla from the High Security Federal Prison in Florence, Colorado, on October 24, 2015.