[1] Niiya attended Kōjōkan Senior High School and began competing in track and field and cross country running while there.

[5] The 2005 World Youth Championships in Athletics in Marrakesh provided her the venue for her first international individual medal, as she claimed the bronze in the 3000 metres.

[7] Niiya repeated both her thirteenth placing and team bronze at the 2006 IAAF World Cross Country Championships junior race,[8] but began to move towards ekiden road running competitions that year.

[9] After graduating from high school she chose to focus on running full-time, working under coach Yoshio Koide, who trained Olympic champion Naoko Takahashi.

[12] Her year ended on a high note as she helped Toyota Industries to their first title at the All Japan Corporate Team Women's Ekiden Championships.

[8][16] Her 2011 season began with two domestic victories at the Chiba and Fukuoka Cross Country races, winning by large margins in both.

[17] At the 2011 IAAF World Cross Country Championships she finished in 26th place, helping Japan to seventh in the women's team rankings.

On the track, she ran a 5000 m best of 15:13.12 minutes in June then claimed the 2011 Asian Athletics Championships silver medal in the event a month later, finishing behind Ethiopian-born Bahraini Tejitu Daba.

At the Chiba International Ekiden in November she ran a stage record on the final leg to help Japan to second place behind Kenya.

"[26][27] She returned to competition in 2018 following rehabilitation for her injury and became the first Japanese winner at the Zatopek 10K in Australia in December 2018, setting a long-term aim to make the team for Japan at the 2020 Summer Olympics in Tokyo.