He is trailed by Chris Brooks (269.025), Jacob Dalton (267.325), Akash Modi (266.225) and Donnell Whittenburg (265.875).
The United States will send five gymnasts to Rio in August based on performances from the previously-held U.S. Championships in Hartford, Connecticut, and this week's Olympic Trials which will conclude on Saturday.
Mikulak struggled early in parallel bars and at one point had to leave the floor because the guards he was wearing on his wrists caused bleeding. But he fought back to secure his position.
Brooks is also on track to make the Rio-bound team despite being the oldest competitor at 29. John Orozco, the 2012 national champion, is hoping to make the cut by showing his value in specific events. He only competed in four, finishing first on value and second on floor.
"Hopefully the Olympic Committee can see my worth in those four events. I think that's my biggest worth to the team," Orozco said.
"I'm going to try to do the same thing (on Saturday) - relax and know I've done everything I can."
U.S. coach Vitaly Marinitch liked what he saw from Orozco.
"Overall, he did really well today," Marinitch said. "He will most likely do four (events) on Saturday. Haven't decided. If needed he might do floor routine but probably not."
Danell Leyva, who claimed bronze in the all-around in the 2012 London Olympics, headed into the trials in 16th place but is now 10th overall.
(Writing by Jahmal Corner in Los Angeles; editing by Sudipto Ganguly)
