Competing against skaters a third of his age, the American-born Macdonald finished 18th out of 22 in the men's park prelims but said it had been "the experience of a lifetime".
"I didn't look at the scores the whole time," said Macdonald, a skateboarding legend who qualified to represent Britain through his Luton-born father.
"I had to ask when we were doing press, what place did I get? I wasn't here for the scores. I was here to be here."
Skating in the fourth and final heat, Macdonald came out wearing a bright yellow helmet and wasting little time in pumping up the crowd.
He fell as he flew out of the bowl to end his first run but he landed a perfect backflip at the end of his second to send the crowd wild.
Macdonald said the spectacular move was was "not a points thing, it's a fun thing."
"I did that for the crowd," he said. "I was trying to represent that this is fun no matter what age you are -- the coolest, funnest, most inclusive thing that you can do."
Macdonald has been skating professionally for decades and even had his own video game, which was released in 2000.
He has won numerous competitions but skateboarding was not included in the Olympics until Tokyo 2020.
Australian 17-year-old Keefer Wilson, who qualified for the final in fifth place, said Macdonald was "a legend to me".
"Being able to skate with him in the Olympics was an unreal thing," he said.
Macdonald grew up near Boston but he moved to San Diego to pursue the skateboarding lifestyle.
He said skateboarding was "the fountain of youth" and that he had no plans to quit yet.
With the Olympics in Los Angeles in 2028, he even refused to rule out another appearance at the Games.
"Let's go," he said. "It's right up the road from me, I live in San Diego."
Macdonald was the oldest competitor in the field but he was joined in the men's park prelims by 49-year-old South African Dallas Oberholzer.
Oberholzer, who also competed when skateboarding made its Olympic debut at the Tokyo Games, said it was important that veterans had a presence at the Paris Games.
"I'm concerned that skateboarding is becoming a bit too serious, that the youngsters might be doing it for ulterior reasons," said Oberholzer, who finished last in the prelims.
"I started skating because it was the best feeling I could imagine."
