Miyahara was born on March 26, 1998 in Kyoto, Japan. Her parents are both doctors. Due to her parents' work, she moved with her family to Houston, Texas when she was five years old and had returned to Kyoto by the age of seven. Miyahara learned English during her time in the United States.
On February 13, 2016, Miyahara graduated from Kansai University High School. She has stated her plans to attend Kansai University and major in literature as well as continuing to study English.
Miyahara began skating while she was living in the United States and came under the guidance of Mie Hamada in Kyoto, Japan at the age of seven.
Miyahara became eligible for junior international competition in the 2011–12 season. She won a silver medal at her Junior Grand Prix debut in Gdańsk, Poland, and finished fifth at her second event in Milan, Italy. Miyahara then won the Japan junior title and placed sixth on the senior level. She was fourth at her first World Junior Championships.
In the 2012–13 season, Miyahara won gold and bronze medals at her JGP events in the United States and Turkey, respectively. Miyahara then won the 2012 Japanese Junior Championships, before placing fifth at the JGP Final in Sochi, Russia.
Miyahara won her first senior national medal, a bronze, at the 2012–13 Japanese Championships, finishing ahead of Akiko Suzuki. She ended her season at the 2013 World Junior Championships where she finished seventh.
Miyahara started the 2013–14 season by winning the Asian Trophy. Debuting on the senior Grand Prix, she finished fifth at the 2013 NHK Trophy, having placed sixth in the short program and fifth in the free skate. At the 2013 Rostelecom Cup, she was sixth in the short program, sixth in the free skate, and fifth overall.
At the 2013–14 Japanese Championships, Miyahara placed fourth in the short, fifth in the free, and fourth overall, behind Akiko Suzuki, Kanako Murakami, and Mao Asada. She was selected to compete at the 2014 Four Continents Championships, where she won the silver medal behind teammate Kanako Murakami after placing fourth in the short and second in the free.
Miyahara finished fourth at the 2014 World Junior Championships — less than a point out of third. She ended her season with a gold medal at the Gardena Spring Trophy.
Miyahara attended a training camp during the summer of 2014 to work with Olympic champion Ilia Kulik on her jumps. She opened the 2014–15 season with a win at the Lombardia Trophy. Competing in the Grand Prix series, she took bronze at the 2014 Skate Canada International after placing fourth in the short program and third in the free skate. She won another bronze medal at 2014 NHK Trophy (fourth in SP, second in FS). With these results, Miyahara was the second alternate for the 2014–15 Grand Prix Final.
At the 2014–15 Japanese Championships, Miyahara placed second in the short and first in the free on her way to her first senior national title. At the 2015 Four Continents Championships, she won silver for the second year in a row, having won the short program and placed second in the free.
Miyahara ranked third in the short, fourth in the free skate, and second overall at the 2015 World Championships, with personal bests in all competition segments. She was awarded the silver medal behind Elizaveta Tuktamysheva. At the 2015 World Team Trophy, she placed fifth individually and third as part of team Japan.
In the summer of 2015, Miyahara traveled to southern California to work with Ilia Kulik for the second year in a row, aiming to add more power to her jumps. She opened her season on the Challenger Series, winning the 2015 U.S. Classic. Turning to the Grand Prix series, she took bronze at the 2015 Skate America before winning gold at the 2015 NHK Trophy, defeating three-time World Champion Mao Asada. These results qualified Miyahara for her first senior Grand Prix Final. She won the silver medal at the event in Barcelona after placing fourth in the short program and second in the free skate with personal bests in the free skate and combined total.
After repeating as the Japanese national champion, Miyahara went on to win gold at the 2016 Four Continents, achieving personal bests in every portion of the competition. She finished fifth at the 2016 World Championships in Boston, the only event of the season where she finished off the podium.
Miyahara opened her season with a gold medal at the 2016 CS U.S. Classic. Turning to the Grand Prix series, she won the bronze medal at the 2016 Skate Canada International, behind Evgenia Medvedeva and Kaetlyn Osmond, and then silver at the 2016 NHK Trophy, behind Anna Pogorilaya. In December, she was awarded the silver medal at the Grand Prix Final in Marseille, being outscored only by Medvedeva. Later that month, she defeated Wakaba Higuchi and Mai Mihara at the Japan Championships to win her third national title.
Due to a stress fracture in her left hip joint, Miyahara withdrew from two February competitions, the 2017 Four Continents Championships and the 2017 Asian Winter Games. To focus on recovery she also decided to withdraw from the 2017 World Championships.
GP: Grand Prix; CS: Challenger Series; JGP: Junior Grand Prix
Small medals for short and free programs awarded only at ISU Championships. At team events, medals awarded for team results only.ISU personal best highlighted in bold