The Irishman was stopped in the 10th round by Mayweather in Las Vegas on Saturday (26 August), as his professional boxing debut ended in defeat. However, the Ultimate Fighting Championship (UFC) star can console himself with his share of the fight's purse, which was reportedly worth £77m.
The figure will put the Dubliner ahead of Ronaldo, whose annual earnings stood at £71m this year, making him the world's highest-paid athlete in the world, ahead of Cleveland Cavaliers star LeBron James and Barcelona icon Lionel Messi.
When Ronaldo and McGregor met last year in a gym the Irishman uses for training, the latter told the Real Madrid star he would soon surpass him at the top of the list.
"You [Ronaldo] were number one on the Forbes list, I was like number 35 or something," McGregor said.
"I'll get up. Maybe next year I'll get you."
Ronaldo, however, dismissed the claim and replied: "I don't think so."
The list is calculated by adding together salaries, bonuses and endorsements, including revenue from sponsorships and appearance fees, but agent fees are not counted. Furthermore, athletes who are inactive for 12 months are removed from the list.
Despite his bumper paycheck, McGregor will not top the list, with Mayweather set to claim the number one spot amid reports his share of the purse from Saturday's fight was worth an eye-watering £233m.
The American, who will walk away from the sport with a 50-0 record, said the fight with McGregor lived up to all expectations and broke the previous pay-per-view record that he shared with Manny Pacquiao.
"We broke all the pay-per-view records," he said at the post-fight press conference. "You will never see me in the ring again. I wanted to break records and do different things. And tonight we broke the Mayweather-Pacquiao record for pay-per-view buys."
However, the Irishman will overtake a number of elite athletes, including Roger Federer, Kevin Durant, Rory McIlroy and Lewis Hamilton.
Furthermore, with Mayweather set to retire from boxing, McGregor will fancy securing top spot soon enough, given his high profile in the UFC circuit and the possibility of him stepping back into the boxing ring.