The 31-year-old Irishman left Donald Cerrone bloodied seconds into the fight by ramming shoulders into his face from the clinch, and when they broke McGregor landed a head kick that wobbled the American and sent him scrambling back to the cage.
Former two-division champion Conor McGregor returned to the Octagon with a stunning knockout of Donald Cerrone in under a minute at UFC 246 on Saturday in his first fight since losing a lightweight title bout to Khabib Nurmagomedov 15 months ago.
The 31-year-old Irishman left 'Cowboy' Cerrone bloodied just seconds into the fight by ramming shoulders into his face from the clinch, and when they broke McGregor landed a head kick that wobbled the American and sent him scrambling back to the cage.
McGregor closed in and rained down strikes as Cerrone covered up, prompting referee Herb Dean to stop the fight after just 40 seconds.
McGregor served a six-month suspension and was fined $50,000 for his role in a brawl at the end of his loss to Nurmagomedov.
He was convicted of assault late last year for punching a man who had refused a shot of whiskey from him in a Dublin pub while in a separate case in 2018 he was charged with three counts of assault and one count of criminal mischief after police said he attacked a vehicle carrying UFC fighters.
He pleaded guilty to a reduced disorderly conduct charge in that case.
After his long absence, McGregor was glad to be back.
"I feel really good, but I came out of here unscathed. I'm in shape. We've got work to do to get back to where I was," McGregor said in his post-fight interview.
The victory was McGregor's first since November 2016 at Madison Square Garden where he beat Eddie Alvarez to win the lightweight title and become the organisation’s first simultaneous two-weight champion.
A crowd of over 19,000 packed out the T-Mobile Arena in Las Vegas on Saturday and an uninspiring undercard was quickly forgotten as McGregor made his way to the Octagon.
Fans waving Irish Tricolours roared support for their man as the introductions took place and McGregor leaped into action as the fight began.
Cerrone had made his name by going toe-to-toe with some of the heaviest hitters in the UFC, giving fans the fights they want to see, but he was caught cold by McGregor's unorthodox shoulder strikes.
"I had never seen anything like that," Cerrone said. "It threw me way off guard. He busted my nose and it started bleeding and then he stepped back and head kicked me.”
Despite a third straight defeat, Cerrone said he had no plans to retire.
“I’m going to keep fighting man. I love this sport.”
McGregor has said he planned to have three UFC fights this year and also revealed he has a lucrative boxing match against Manny Pacquiao in the pipeline.