Hernandez’s elite career started in 2012 at the U.S. Classic, where she placed 11th in the junior division at the age of 12. Through the Classic, she qualified to the National Championships in St. Louis, where she placed 21st after two days of competition.

Hernandez won the junior all-around at the 2013 Elite National Qualifier. Following a national training camp, in July 2013 Hernandez was added to the U.S. junior national team. She then went to Chicago for the U.S. Classic, where she placed sixth all-around and won the floor exercise title. At the National Championships in August, she won the silver medal in the junior all-around competition with a total score of 116.650, behind Bailie Key. She also placed second on uneven bars and floor exercise, and tied for third on beam with Alexis Vasquez.

In September, Hernandez was selected to represent the U.S. at the Junior Japan International in Yokohama. She scored 56.750 to win the bronze medal in the all-around. She also took third on vault, fourth on floor exercise, and sixth on balance beam. Later in 2013, she competed at the International Junior Mexican Cup in Acapulco and helped the U.S. team—Hernandez, Key, Veronica Hults, and Emily Gaskins—win the gold medal. She also took second place in the all-around behind Key.

In early 2014, Hernandez fractured her wrist when she slipped off the beam in a training session. She returned to competition soon after the injury but then suffered a torn patellar tendon and dislocated kneecap, resulting in six months out of gymnastics. She resumed training in the fall and attended the final U.S. training camp of the year in November.

In 2014, Hernandez committed to the University of Florida to compete on the Florida Gators gymnastics team in the NCAA. However, she decided to forgo NCAA eligibility and become a professional athlete on August 3, 2016, in the lead-up to the Olympic Games.

At the 2015 U.S. Classic in July, Hernandez won the junior all-around title with a score of 58.450, as well as winning vault (14.900) and uneven bars (15.000). She placed third on balance beam (14.200) and floor exercise (14.350, tied with Deanne Soza).

Hernandez made her senior debut in 2016. In March, she competed at the City of Jesolo Trophy in Jesolo, Italy, earning the bronze medal in the all-around with a score of 58.550, behind two U.S. teammates, fellow first-year senior Ragan Smith and Gabby Douglas, the 2012 Olympic all-around champion. She also earned a silver medal on vault and a gold medal on balance beam, ahead of Smith and 2012 Olympian Aly Raisman.

Hernandez was revealed as one of the celebrities competing on season 23 of Dancing with the Stars on August 30, 2016. She was partnered with professional dancer Valentin Chmerkovskiy. The couple won the Mirrorball Trophy on the episode that aired November 22, 2016. At 16, Hernandez is currently the show’s youngest winner.

On November 24, 2016, she appeared on the 90th anniversary of the Macy’s Thanksgiving Day Parade. She was on the Spirit of America float.

On October 31, 2016, Hernandez announced her book I Got This: To Gold and Beyond on Instagram. The release date was January 24, 2017. On February 1, Hernandez shared on Instagram and Twitter that she became a New York Times Best Selling Author for the book.

In November 2017, Hernandez returned to the 25th season in week eight, to participate in a trio jive with Victoria Arlen and her professional partner Valentin Chmerkovskiy.

Hernandez was born in Old Bridge Township, New Jersey, the daughter of Wanda and Anthony Hernandez. She attended Abeka Academy High School in Old Bridge, New Jersey. She has a sister, Jelysa, and a brother, Marcus, who graduated from Rutgers University in 2018. She is of Puerto Rican descent.

After the Olympics, Hernandez appeared on the television show Dancing with the Stars. She continued appearing on television and in the media throughout the next four years, as a show host and voice actor as well as a celebrity guest. She returned to gymnastics training in October 2018, after two years off. Although Hernandez previously trained in New Jersey at MG Elite, for her comeback she chose to switch gymnastics clubs and train at Gym-Max in California with coaches Jenny Zhang and Howie Liang. In August 2019, she said that she was training five hours per day, six days a week. Hernandez was invited to the national team training camp held November 15–18, 2019 after spectating at the US Championships in August and speaking to team coordinator Tom Forster while there.

On July 15, 2018, Hernandez announced on Instagram that she will be co-hosting American Ninja Warrior Junior.

On September 11, 2018, it was announced that Hernandez would be joining Jane Lynch and Tim Gunn in the Nickelodeon animated mini-series Middle School Moguls, voicing a character named Valeria.

In 2018, Hernandez published a similar children’s book for younger readers titled She’s Got This, with illustrations by Nina Mata.

In January 2020, Hernandez said that she was focusing on meets beginning in late May 2020: the U.S. Classic, USA National Championships, and the US Olympic Trials. Hernandez did not attend the February national team training camp, despite being invited. As the February camp was a selection for spring international meets, Hernandez could not participate in competitions held in March and April 2020. According to team coordinator Tom Forster, Hernandez accepted an invitation to the April camp. On April 29, Hernandez’s former coach Maggie Haney was suspended by USA Gymnastics for eight years due to abusive conduct. Hernandez testified against Haney at the USA Gymnastics hearing, and posted a message about her experiences on her social media without naming the coach. Haney publicly humiliated Hernandez for her weight, leading Hernandez to binge and purge. She also forced Hernandez to train and compete while injured. As of 2020, Hernandez still struggles with depression and disordered eating that began due to Haney’s abuse. Within the gymnastics community, Haney’s suspension was seen as progress for USA Gymnastics, and fans widely supported Hernandez online.