On the first day of competition at the 2022 Junior Pan Pacific Swimming Championships, held in August at the Veterans Memorial Aquatic Center in Honolulu, United States, Mitsui won the gold medal in the 200 metre butterfly with a Championships record and personal best time of 2:07.82, which lowered the mark 0.66 seconds from the 2:08.48 set by Cassidy Bayer of the United States six years earlier.

[2][3] The following day, she placed thirty-second in the 100 metre freestyle preliminaries with a personal best time of 57.79 seconds and did not advance to the evening finals.

[7] In the evening final, she placed sixth with a time of 2:05.40, finishing after a gold-silver sweep by two American swimmers and 5.79 seconds behind the world record of 1:59.61 set by Mireia Belmonte of Spain in 2014.

[8][9][10] In the preliminaries of the 100 metre butterfly on day one at the 2023 Japan Swim in April in Tokyo, Mitsui ranked sixth with a time of 58.96 seconds.

[14][15] In May, at the first stop of the 2023 Mare Nostrum tour, in Canet-en-Roussillon, France, she won the gold medal in the 200 metre butterfly with a time of 2:09.13.