"I kind of laughed. It's almost absurd I was able to do it again," said Ervin of his immediate reaction to seeing his name with the number one against it.
France's Florent Manaudou, the defending champion, finished second and just 0.01 seconds behind the 35-year-old veteran, who won in 21.40 seconds in a splash and dash down the pool.
"It was a very disappointing race. I flagged at the end. I did exactly the opposite of what I did four years ago," said Manaudou.
"Tonight I was hungry and I really wanted to win. Unfortunately, I wasn't the best today. I was the best in the heats but tonight he was better.
"I felt after 15 meters that the race wasn't going well, that I was too deep in the water and I couldn't get any speed. It's hard to come back after that."
The entire field was separated by just 0.68 seconds, with Ben Proud of Britain finishing fourth.
He later sold that medal to raise money for relief efforts after the 2004 Indian Ocean tsunami.
Asked earlier in the week what he would do if he won another, Ervin had replied: "I don't know. I'm living in the moment, man."
Heavily tattooed, and as the oldest man on the US swim team making 31-year-old Michael Phelps look comparatively young, the Californian gave up competitive swimming in 2003 but returned for the 2012 Games in London.
Friday's medal was his second gold from Rio, the first coming in Monday's 4x100 freestyle. He also has a 4x200 freestyle silver from 2000.
"If anything, I was a little bit slower than I thought I was going to be," he said.