Johnson, a retired gym teacher and coach from San Jose, Calif., hit her head on concrete at the 20-mile mark, but got to her feet and finished the 26.2-mile run in just under 8 hours, Long Island's Newsday reported.
Diana Boydston, 54, also of San Jose, told the newspaper Tuesday she learned Monday her mother had taken a tumble and was taken to a medical tent along the course.
"They checked her out and she was coherent and seemed fine," Boydston said. "Very conversant. With my mom, if she thinks she's feeling well, she's going to keep going."
Johnson and her sister, Faith Anderson, had appeared on the "Today" show Monday morning and even then described her injury as "a scratch."
"It looks worse than it really is," she said.
But after returning to her hotel and taking a nap, she was unresponsive and was taken to a Manhattan hospital where she died a short time later.
Marathon officials said Johnson, who didn't take up running until she was 59, had completed her 25th straight marathon, six times winning her age group.
"At least she was running, the way she wanted to go," Boydston told the New York Daily News.
Johnson had said just that Saturday when she posed for photos with much younger runners.
"I always say I'm going to run until I drop," she said. "I'm going to die in my tennis shoes. I just don't know when I'm going to quit."