[18][19] Mihara made her senior international debut in early August 2015, winning the Asian Open ahead of Riona Kato.

Ranked second in the short program and first in the free skate, she won the gold medal ahead of Russia's Elizaveta Tuktamysheva.

[26] In February 2017, she placed fourth in the short and first in the free at the Four Continents Championships in Gangneung, South Korea, outscoring Canada's Gabrielle Daleman by 3.94 points for the gold medal.

[40] Poor health forced Mihara to withdraw from both of her Grand Prix assignments, the 2019 Skate Canada International and 2019 Cup of China.

[42] Mihara returned to competition domestically, winning the bronze medal at the Kinki Regional Championships and then silver at the Western Sectionals.

[43] With the COVID-19 pandemic affecting international travel, the ISU designed the Grand Prix primarily based on geographic location, and Mihara was one of eleven Japanese skaters assigned to the 2020 NHK Trophy alongside South Korean You Young.

[44] Mihara was third in the free skate, rising to fourth place overall, slightly more than four points behind bronze medalist Rino Matsuike.

[47] Beginning the season at the Olympic test event, the 2021 CS Asian Open Trophy, Mihara won the gold medal.

[48] She was initially assigned to only one Grand Prix event, the 2021 Cup of China, which was subsequently replaced by the 2021 Gran Premio d'Italia.

[49] Upon Rika Kihira's withdrawal from the 2021 Skate Canada International due to ankle injury, Mihara was named to replace her.

[51] Competing in Italy the following week, she again placed fourth, setting new personal bests in the free skate and total score.

She was fifth as well in the free skate segment, making a notable error by performing only a single Axel instead of a planned double in combination with a triple toe loop.

Despite an error on the triple flip, she won the short program, finishing 0.17 points ahead of American skater Isabeau Levito.

She won the free program by a wider margin after a clean skate, taking her first Grand Prix gold medal.

[56] Both she and Hendrickx made errors in the free skate segment, but Mihara took first overall, earning her second gold medal and qualifying to the Grand Prix Final for the first time in her career.

[60] Mihara went on to finish first in what the ISU itself characterized as a "turbulent" free skate segment, despite underrotating one jump and falling out of another, while frontrunner Sakamoto dropped off the podium entirely.

[63] She was second in the free skate as well, albeit 10.03 points behind Sakamoto, winning the silver medal and standing on the Japanese national podium for the first time in six years.

[64] Competing at her second Winter World University Games (formerly Universiade), this time held in Lake Placid, Mihara finished second in the short program behind Sakamoto.

Afterwards, Mihara said she was " so frustrated, so much that I can't cry," adding that "one of my weaknesses is making mistakes in the last important event.

I feel that I still have a long way to go to become a top athlete"[68][69] Mihara was named to compete at the World Team Trophy for the first time in six years, and said she was hoping to end her season on a high note following frustration with her performance in Saitama.

[73] Mihara was scheduled to begin the season at the 2023 CS Finlandia Trophy in October, but a "nagging" right ankle injury forced her to withdraw from that event.

She subsequently withdrew also from her first Grand Prix assignment, the 2023 Cup of China, saying "I have done everything I can to treat and prepare for competition but have had to make the frustrating decision to withdraw.

Reflecting on her season, Mihara said she "didn't have many satisfying run-throughs" in the leadup to the championships, adding "I'm very relieved that I made it to the end thanks to my coaches and everyone's support.

[81] In late December, Mihara competed at the 2024–25 Japan Championships, where she placed twenty-third in the short program after falling on two jump attempts.