A jury unanimously convicted officer Randy Roedema of criminally negligent homicide and third-degree assault.
Another officer, Jason Rosenblatt, facing the same charges was acquitted.
Both men were accused of using excessive force and ignoring the unarmed black man’s repeated cries of “I can’t breathe”.
Under Colorado law, Roedema now faces possible prison time and a fine.
He was suspended without pay from the Aurora Police Department, while Mr Rosenblatt was fired in 2020.
On 24 August 2019, Mr McClain, a 23-year-old autistic massage therapist, was walking home from a corner shop when three white police officers confronted him after an emergency caller reported a “sketchy” person in the Denver suburb of Aurora.
He was wearing a balaclava, which his family has said protected him from chronic chills due to his anaemia.
Listening to music on his headphones, he had initially ignored officers’ calls for him to stop walking.
Body camera footage of the incident showed him pleading with officers that he did nothing wrong, before they wrestled him to the ground and placed him in a chokehold.
After officers called for assistance, a medic injected Mr McClain with ketamine to sedate him.
Mr McClain went limp, stopped breathing and never regained consciousness. He was declared brain-dead three days later.
A key focus of the two officers’ trial was how Mr McClain died.
Meanwhile, lawyers for the officers argued they had no choice but to forcefully subdue him after he resisted them.
They also argued that it was the ketamine that killed Mr McClain, and that the paramedics were responsible for his death.
An autopsy report in September 2022 concluded that he had died from “complications of ketamine administration following forcible restraint”.
In a statement on social media after Thursday’s verdict, Aurora Police Department Chief Art Acevedo said: “We respect the verdict handed by the jury.”
Mr McClain was declared dead on 27 August 2019, but his death faced renewed scrutiny after Mr Floyd’s murder.
Several others have also been charged with his death, including a third officer, Nathan Woodyard, and medics Jeremy Cooper and Peter Cichuniec.
Roedema is scheduled to be sentenced in January.
Elaine Hadley is a dedicated journalist covering the ever-evolving landscape of U.S. news. With a keen interest in politics and a commitment to uncovering the truth, she provides insightful commentary and in-depth analysis on domestic issues. When not reporting, Elaine enjoys exploring the diverse cultures and landscapes of the United States.