[20] 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.

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 won her first senior national medal, a bronze, at the Japanese Championships, finishing ahead of Akiko Suzuki.

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.

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 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.

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.

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.

[22] 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.

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.

[24] Due to a stress fracture in her left pelvic girdle, Miyahara withdrew from two February competitions, the 2017 Four Continents Championships and the 2017 Asian Winter Games.

[28] At the 2017 NHK Trophy she finished 5th after underrotating jumps in both nights, but rallied three weeks later to win gold at the 2017 Skate America.

At the 2017 Japan Championships, Miyahara was crowned the national champion for the fourth time after placing second in the short program and first in the free skate.

[31] In Taipei, Miyahara was first after the short program, third after the free and won the bronze medal overall, behind teammates Kaori Sakamoto and Mai Mihara, contributing to a Japanese sweep of the podium.

[32] At the Olympics, Miyahara was assigned to participate in the ladies' short program in the Team Event, with her teammate Kaori Sakamoto chosen to compete in the free skating.

[33] In the individual event, Miyahara scored new personal bests with clean performances, placing fourth in both segments of the competition and overall.

[34][35] At the World Championships, Miyahara was in third after the short program, with a score of 74.36 after underrotating a triple toe loop in her jump combination.

[38] At her second Grand Prix event, the 2018 NHK Trophy, Miyahara placed second in both programs, narrowly winning the silver medal ahead of Elizaveta Tuktamysheva and behind compatriot Rika Kihira.

"[39] Her results qualified her for the 2018–19 Grand Prix Final, where she finished sixth, following a combination error in the short program and several underrotated jumps in the free skate.

Several errors lead to her placing fourth in the free skate, dropping to the bronze medal position overall behind Sakamoto and Kihira.

[47] Competing the following week at the 2019 Rostelecom Cup, Miyahara placed sixth in the short program after doubling a planned triple Lutz and managing only a triple-double jump combination.

[50] She was sixth in the free skate after falling once and underrotating or downgrading eight jumps, and dropped to fourth place overall, her first time finishing off the podium at her national championships since 2013.

[53] Miyahara was given a bye to the Japan Championships due to the COVID-19 pandemic making it impractical for her to travel back from her training location in Canada for lesser domestic events.

"[60] Miyahara withdrew from the 2021 CS Autumn Classic International, and thus did not participate in a Challenger event prior to the beginning of the Grand Prix.