He is a six-time U.S. national all-around champion (2013–2016, 2018–2019), the 2018 World bronze medalist on the horizontal bar, and a three-time Olympian (2012, 2016, and 2020).

Mikulak was born in Huntington Beach, California,[2] and attended Corona del Mar High School.

Mikulak attended SCATS Gymnastics and was a United States Junior National team member from 2007 to 2010.

[3][5] As a Junior Elite gymnast, Mikulak won the silver medal in the all-around three years in a row in the 14–15-year-old division at the 2007, 2008, and 2009 U.S. National Championships.

[3] As a sophomore in 2012, he won the Big Ten Conference title on the horizontal bar and pommel horse and placed second in the all-around.

[12] In March 2014 at the American Cup in Greensboro, North Carolina, Mikulak won the all-around title.

In March, Mikulak, along with Simone Biles, MyKayla Skinner, and Donnell Whittenburg, represented the U.S. against a strong international field at the 2015 AT&T American Cup at AT&T Stadium in Arlington, Texas.

On July 11, 2015, Mikulak led the U.S. men's gymnastics team to their first gold medal at the Pan-American Games in twenty years.

[14] Due to a partially torn Achilles tendon injury, he was unable to participate at the 2015 World Championships in Glasgow, Scotland.

[15][16] On December 17, 2015, Mikulak and two-time World medalist Donnell Whittenburg accepted invitations to represent the U.S. at the 2016 American Cup in Newark, New Jersey, on March 5, 2016.

On February 21, 2016, Mikulak won the all-around title at the 2016 Winter Cup in Las Vegas, with a two-day score of 177.50.

In March, Mikulak finished fourth at the American Cup, his first international competition since recovering from a torn Achilles tendon.

He was named to the five-man U.S. Olympic men's gymnastics team along with Chris Brooks, John Orozco (who was replaced by Danell Leyva due to injury), Jake Dalton, and Alex Naddour.

His streak of four U.S. national all-around titles was snapped, as he was still recovering from an Achilles tendon injury suffered in February at the Winter Cup.

In October, he competed only on the horizontal bar at the 2017 World Championships in Montreal, Canada, as he was still not fully back from his injury.

As Mikulak was recovering from injury, he continued to host and participate in numerous clinics and workshops for aspiring gymnasts.

Following the Winter Cup, the top six ranked all-around gymnasts including Mikulak automatically qualified for the 2018 U.S. national team.

In April, he competed at the 2018 Pacific Rim Championships and won the gold medal with the U.S. team and the all-around title.

Mikulak is the first male gymnast to win five U.S. national titles since Blaine Wilson won five straight crowns from 1996 to 2000.

In April, Mikulak competed at the 2019 FIG All-Around World Cup in Tokyo, Japan, an individual all-around-only event in which he was the defending bronze medalist.

As a result, Mikulak was chosen by the selection committee to represent the United States at the 2020 Summer Olympics alongside Brody Malone, Yul Moldauer, and Shane Wiskus.

He had posted the highest qualifying score in the field, but he did not win a medal in the event finals, even though several other contenders had made errors.