Donald Beardslee was born in 1943 in St. Louis, Missouri. He was the oldest of three children. When he was 11, his father passed away, which greatly affected his family. At 15, Beardslee was sent to Western Military Academy in Alton, Illinois, a military school. Later, he attended Southwest High School in St. Louis and graduated in June 1962.

In August 1962, at the age of 19, Beardslee joined the U.S. Air Force and served for four years as an aircraft mechanic. During his service in 1965, he and another airman were caught trying to steal a vehicle. As a result, he was sentenced to a work farm in Minnesota. While there, he suffered a serious injury when a falling tree hit him, causing a head fracture and leaving him in a coma for several days.

In 1969 in Missouri, Donald Beardslee met Laura Griffin at a bar. After drinking and dancing, they went to her place. That night, he choked, stabbed, and drowned her. Beardslee later turned himself in and confessed. He pleaded guilty to second-degree murder and was sentenced to 19 years but served only seven before being released on parole in 1977. He never explained why he committed the crime.

Four years after his release, while still on parole, Beardslee was arrested in California for the drug-related murders of Patty Geddling and Stacie Benjamin on April 5, 1981. A drug deal involving his roommate's friends had gone wrong, leading to the murders. Beardslee and another man, Bill Forrester, shot Geddling, and Beardslee later slashed Benjamin's throat. Beardslee's phone number was found near Geddling's body, and he confessed when the police called.

At the time of these murders, Beardslee was still on parole for the 1969 killing, making him eligible for the death penalty. Frank Rutherford, the drug dealer who devised the scheme, was sentenced to life without parole and died in 2003. Beardslee's roommate, Ricarda Sue (Rickie) Soria, pleaded no contest to second-degree murder in exchange for her testimony and was sentenced to 15 years to life, later paroled in 2015. Bill Forrester was acquitted, and charges against another person were dropped.

Beardslee was convicted of first-degree murder and sentenced to death. His defense tried to appeal, arguing inadequate defense and mental impairment due to a head injury, but the prosecutors noted his above-average intelligence. Beardslee was executed by lethal injection on January 19, 2005, becoming the first prisoner executed in California since Governor Arnold Schwarzenegger took office. Beardslee had no final words, and his request for clemency was denied by Schwarzenegger, who stated that Beardslee knew the difference between right and wrong. While on parole, Beardslee attended the College of San Mateo.