Daniel Heckert, a runner who had finished the half marathon, spotted Mazur and Robertson approaching the cheer station in the last couple of kilometres, hand-in-hand.
"I just ran over to grab my phone as they were coming," he told CBS Pittsburgh.
"I just wanted to capture the moment that showed these two ladies were going to finish the same race that [the] winner did."
His video quickly went viral on Facebook.
It took the two women seven hours and 22 minutes to finish the marathon and they came absolute last - but they never gave up.
The marathon announcer thanked them for "being an inspiration to all of us".
It was Mazur's 12th marathon and Robertson's first. They say they are now "life-long friends".
"It wasn't about gender, it wasn't about race, it wasn't about religion - none of that mattered," Robertson told CBS Pittsburgh.
"We were just two individuals, two humans, who had a goal in mind and we leaned on each other to get there."