The 25-year-old 200 and 400 metre runner ran the first leg of America's heat on a blazing hot day inside London's Olympic stadium and although he finished with a limp there was no obvious sign of the pain he was feeling.
"I got out pretty slow, but I picked it up and when I got to the 100-metre mark it felt weird. As soon as I took the first step past the 200-metre mark, I felt it break. I heard it. I even put out a little war cry, but the crowd was so loud you couldn't hear it," Mitchell said.
"I knew if I finished strong we could still get it (the baton) around. I saw Josh Mance motioning me in for me to hand it off to him, which lifted me. I didn't want to let those three guys down, or the team down, so I just ran on it. It hurt so bad."
Mitchell's decision to go through the pain barrier allowed his U.S. team mates to sprint to a joint finish with the Bahamas in identical times of 2 minutes 58.87 seconds, the fastest ever run in the first round of the relay at the Olympic Games.
Mitchell, who ran his leg in 46.1 seconds, said that he had slipped on a stairway this week in the Olympic village but had completed workouts since, including a warm-up for Thursday's race.
After the race an x-ray revealed he had broken his left fibula bone.
The US team was already without 2008 Olympic 400m champion LaShawn Merritt, whose title defence lasted only moments last Saturday when he pulled up with hamstring problems.
The US, one of the favourites for gold, will name their final line-up tomorrow.
