"I said yes, which was basically a lie," he said. A seventh-generation South African, he didn't want to take any chances by revealing his citizenship.
But he kept running, and running well.
Finally, the IAAF, the world's governing body for track and field, contacted USA track and field.
He was promptly banned from competing, which affected deals with his sponsors. Because the ban wasn't legal, he considered entering more events. He drove to Atlanta in 1985 to run in the road race. The media heard about the ban, and the fact that he was actually South African, and began publicizing his entry.
Trying to stop him from running, the IAAF told other runners that if they competed against him, they would be banned. Peachtree organizers told him he couldn't run.
Not wanting to jeopardize the careers of his friends and competitors, he quit entering events.
At the time, he felt it was unfair that athletes were being used as targets, but looking back, he said he understands why it had to be done.
"We all have to make sacrifices," he said.
He said he received a lot of support in Atlanta, not because they agreed with South Africa's government, but because they didn't agree with anyone being banned from a race that is annually held on Independence Day.
Eventually, he was told that if committed to becoming a U.S. citizen, he could run again.
He began the process, started competing again in 1988 and became a citizen in 1992. However, he never returned to Atlanta other than for the 1996 Summer Olympics.
Johnson's wife, Andrea, thought it might be a nice gift to give him another chance to run the Peachtree Road Race. She called race organizers, who jumped at the chance to invite him back.
He drove from Indianapolis on Saturday and watched tape of his last race in the city. He said a lot has changed since then, laughing at the mustaches sported by the men in the field.
No longer a competitive runner, he took his time Sunday, enjoying the bands that played alongside the route and watching the people.
As for his country, Johnson said it has come a long way, though he said there's still much to do. Apartheid ended in 1994, when the first multi-racial elections were held.
Growing up, he said he, like many of his countrymen, didn't realize the depths of what the politicians in the country were doing.
"Truth and reconciliation is really coming out as we realize how bad it was," he said. "I'm just happy that I could do my part to help end apartheid."