"I figured it's what almost any person would've done in that situation," Mitchell told the Associated Press.
The relay final takes place on Friday.
Mitchell said he felt weird around the 100-meter mark and felt a popping in his left leg.
Then around the 200-meter 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 told Reuters.
"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."
On Day 13 of the London Olympics, the AP reported he finished the first lap in 46.1 seconds. His decision helped the Americans tie for first with the Bahamas in identical time: 2 minutes, 58.87 seconds.